There are many sellers of...um...undergarments, but not usually the same sizes are you might be used to in more amply-endowed parts of the world. Most ladies in Korea are A or B cups with a few Cs so good luck finding many in the larger sizes.
Also, almost all Korean women (even those in the 'thong' throng of young, scanty-dressers) seem to prefer 'granny' underwear so if you're a minimalist you'd better bring some from home.
Toiletries and Medical Supplies:
I said it wasn't here, but it is.
Go to Hannam Supermarket, go to Hannam station on line 1 and head out the subway to the street (there is only one exit). Turn right, and follow the street until you reach a large intersection with an overpass (this is the stretch of highway between Namsan and Hannam Bridge). Cross the highway at the intersection, and walk along it to the left for about 5-10 minutes. At this point, you can spot the large blue Volvo sign at the top of the building. The supermarket is in the basement, down some steps just after the Volvo building.
It's actually a few different stores. The one at the back is larger and has more things, but is more expensive, while the little store at the front is cheaper. There's also a separate deli counter for meats and cheeses.
Luggage and bags:
Backpacking in winter is no joke in Korea. Yes, there's no reason to giggle when you're carrying 10kg worth of clothes and equipment at -10c. So get your act right and get a good backpack to hold your essentials. Don't get cheap and buy your bag from a street vendor. Trust me, you'll lose your sanity when your bag bursts in the middle of a subway station. Picture yourself saying this in Korean...'Godammn it, would you move aside, you're stepping on my underpants. '
Clothing/Shoes/Weather Gear: Layering is the trick to lighthen your load and keeping yourself warm.There's no point bringing all your bulky sweaters from home unless you want to be the Michelin Man. Wear one thin wolly layer atop another and if your budget allows, go for lightweight sweaters made of Thinsulate or Dupont. You'll be as snug as a bug under those materials. As for shoes, go for a good pair of waterproof boots and socks made from lamb's wool.
Ladies: Don't bother with heels , they'll seriously impair your ability to scale those mountains and you'll return from Korea with a leg cast for a souvenir.
Toiletries and Medical Supplies: Bring aspirins for impending colds and diarrhoea.
Ladies : I read in some archaic guide books that there are no tampons here in Korea. What a load of bull****. They're available at all convenience stores over here.
Photo Equipment: Digital Cameras are ideal for backpacking as they're feathers compared to the ancient bricks. All the photos in my webpages were taken by my handy Sony digishot. It's lightweight and it has survived subzero temps and underwater shots at 20m.
Camping/Beach/Outdoor Gear: A good water-proof parka will withstand light rain and snow. There's no need to bring your bikini if you're going to a hot spring. You'll stand out like an Eskimo in Miami. Koreans bathe in their skins, just like the Swedes.
(Bring one if you're going to the touristy Waterpia though )
Miscellaneous: There's no point buying heat packs from home unless you're really a tropical fellow. Just give your body a couple of days to acclimatise to the weather. I bought 10 or more packs but they served more as deadweights as I didn't use them. Also, they cost a bomb in Singapore and a fraction in Korea...arrgh...
Pack as lightly as you can. For autumn...it is best to try layering than bringing all those bulky sweatshirts and jackets.
Bring good, sturdy and comfy shoes for everyday walking.
Toiletries and Medical Supplies: Eye drop - I didn't realize that it is necessary to bring one. I visited in autumn and my eyes felt so dry and itchy all the time.
Plastic Strips (Band-Aids) - bring lots and lots of these medical wonder ;0). I'm prone to blisters but then I never thought I'd be doing loooong walks in Korea. Before end of my trip, I have used up at least half box of strips.
Medicines - When I had to buy a pain reliever in one of the pharmacies, I had difficulty getting one because the pharmacist couldn't understand what I was saying. I tried different kinds of tricks, saying the brand name, generic name, saying body pain, fever, motioning, etc. In the end, I asked for a pen and pencil and bingo! he managed to understand me. The only problem is that, I could not take the medicine before I actually checked from my Korean friend if the man gave me the pill that I wanted.
Lip balm - your lips could easily get chapped with the cold weather.
Oil, lotion - for moisturizing, prevents you from having those dry and itchy skin.
Photo Equipment: For Digital Camera Users, bring a high capacity memory cards as there would be many photo opportunities in Seoul. It is also good to bring blank CDs as you can burn your photos daily to free those precious memory cards.
Miscellaneous: Note: all of the above are of course available in Seoul. Bring them nonetheless to save time, effort and money.
Toiletries and Medical Supplies:
You should always carry some tissues or toilet paper with you. Toilet paper is usually not provided in public restrooms. Also take care not to throw the paper in the toilet. Use the garbage can instead.
Miscellaneous: And I just wanna say... Have fun with the Asian Toilets! They are really hard to use. Whenever possible, try to use a western Toilet!
Luggage and bags:
I highly recommend packing lightly, and if you don't have access to on-base USPS mailing, an extra suitcase per person. The shopping in Seoul is out of this world, and you will buy a wardrobe a day, if you don't watch yourself. If you have access to the Yongsan Camp, consider yourself lucky, and start sending back your plunders as soon as you can.
Clothing/Shoes/Weather Gear: I was in Seoul in the Fall-Spring. My advice? bring a cheap, warm jacket and as few outfits as possible, because when you get there, you will no doubt see that American concepts of style and Korean fashion don't mix, because they dress 10 times better at all times. Go to Myeongdong, Dongdaemun, &c and get some nice looking clothes! Bring one pair of shoes, because, again, you are going to end up buying shoes here, for dirt-cheap prices.
Toiletries and Medical Supplies: STOCK UP on medicine. It is sometimes difficult to find medicine if they don't have a repackaged US brand where you are shopping. I cannot stress enough- if you don't have access to a US base, BRING ENOUGH DEODERANT for your entire trip. I didn't believe it at first, when I saw this gigantic, multi-floor Wal Mart with NO DEODERANT.
Photo Equipment: I used a Canon EOS 10D. I would recommend the same, as it is a bargain since it has been replaced by the 20D. My problem- I brought only a 50mm lens. You HAVE to have a wide angle, you HAVE to. It has ruined my photos almost the whole time I've been here, but I have gotten really good at the 50mm (which is more like 75mm when placed on a small-sensor DSLR). I highly recommend the Tamron 19-35... it is a bargain on ebay or new in the states, but in Korea, expect to pay $300 or more even the CHEAPEST lenses. Seriously, do not use a cheap camera. I would recommend using a 10D as well as a Canon Powershot S60, also a bargain because it was replaced, to keep in your pocket when lugging around the Canon is too much trouble. Also, invest in a decent, light-weight tripod. I learned the hardway that night shots, some of the most beautiful in Seoul, can escape even a decent amateur photographer without a tripod.
Camping/Beach/Outdoor Gear: It is cold in the winter. Just keep it in mind. Down south, it hovers about 10 degrees F hotter than Seoul.
Miscellaneous: My personal number-one item is an Apple laptop. I am on tour with three other people. Two have Apples, two have PCs. The PC guy and girl have constantly had trouble with configs in hotels, public wireless access points, &c. It is getting silly. I build PCs and know how to handle networking, and I've still encountered problems on their PCs trying to negotiate a connection from time-to-time here. You will need a laptop, because it is very easy to find a wireless connection in Seoul, and you will constantly want to read Virtual Tourist, or try to find translations for Korean or basic phrases. Trust me, Seoul is overwhelming. Don't depend on your hotel to have computers that even work, if at all, or working internet or wireless, if they offer it. A laptop puts you in control.
If you're over a size 8- you can't buy shoes here girls! so bring what you'll need... however many that may be...
Also women's underwear and bras are quite differently sized. I'm a western size 6 and they are too small for me... unless you like granny underwear! haha
Toiletries and Medical Supplies: Tylonol & Advil here aren't as strong so bring those. As well as cold as flu medication.
Deoderent as we know it hasn't broken through heere yet. Tampons are beginning to but hard to find sometimes. Also you never know which moiturizers have whitening in them so bring that too. Also can't buy woman's shaving cream. (men's no prob)
Toiletries and Medical Supplies:
The hardest thing there is to find here is deodrant make sure that you stock up before you come. So you don't run out if your here for a long time. Then what would you do just walk around stinking. Also for the ladies i hear they don't use tampons so if thats what you use you better bring enough.
Everything else you wanted to bring but was not sure you can buy here. They have every thing you can buy at home and more. Make sure you underpack your bags so you can bring home some souvenirs.
Luggage and bags:
Korea has stairs, lots of them. Therefore it's best to leave the gigantic wheeled bag at home if you are going to be relying on moving around the city by subway or walking many places with your stuff.
Clothing/Shoes/Weather Gear: Summer is the rainy season. Bring shoes that can get wet or at least dry out quickly if you are planning on traveling in June and July. No need to bring an umbrella though, you can buy one here for as little as 3,000 won.
Clothing sizes here aren't the same as the US, so if you wear a size larger than medium in the US, it's best to not plan on buying tons of clothes in Korea. It's a good idea to figure out your shoe size and clothes size before you go shopping here, as there are not always places to try things on.
Toiletries and Medical Supplies: As mentioned by others, deodorant is not easy to find here. However, I did see some in E-mart in Wangsimni station. Also, it's a good idea to bring any pain and cold medicines you might need, unless you can read Korean. It is not fun to figure out dosage instructions in another language when you are ill.
I haven't found anywhere (though I haven't really looked) that sells anti-bacterial hand sanitizer. This is nice to have when the sink in the bathroom is either dirty of nonexistent.
Camping/Beach/Outdoor Gear: Like hiking? Then bring a pair of boots with you. If you should forget your hiking stuff, no worries. Korea has tons of shops with both western and Korean outdoor equipment.
Miscellaneous: If you are staying in a dorm or other place that doesn't provide bed linens; bring your own sheets. Quilted type blankets are easy to find and relatively cheap at Dongdaemun or the large stores (E-mart, Homeplus, etc). However, western sheets are often expensive (>30,000 won) and the selection is limited.
Again, if you're staying for a longer amount of time, know that you'll pay a premium for groceries such as peanut butter, cheese and other imported western products. I'm not sure it's worth it to pack a suitcase of peanut butter, but it's just something to note.
Luggage and bags:
Alarm clock is a must too, so you don't waste your day or miss your flight if you sleep in too long;)
Clothing/Shoes/Weather Gear: Bring a water proof coat if you are travelling to Korea in the fall or winter. Also, it's always a good idea to bring some comfortable shoes to walk in during the day.
Toiletries and Medical Supplies: Waterless soap, it saves you when you decide to eat someplace on the street and don't have a place to wash your hands.
I didn't exactly intend on visiting Korea per say and February is not the optimum time to go. Although the palaces may look beatiful covered with white snow, it is cold and transportation seemed to shut down with the storm. It was about -5 degrees which is rather mild to some Canadian and Scandanavian standards but a good winter coat and boots are necessary!!! We were slipping and sliding all over the place.
Toiletries and Medical Supplies: Visine!!!! Maybe it is just Asiana or maybe just all long distance jets, but in an effort to sedate all the passengers (at an unreasonable local hour), the heat was turned up and the oxygen was lowered. My eyes were sooooo dry and bloodshot and it took days for one to clear up. I would have loved some visine then.
Miscellaneous: It doesnt matter what kind of travel, bring a Korean dictionary and learn some basics before arrival. It is unreasonable to expect Koreans to all speak english and I found communication was very difficult when I found myself in a dire situation. Between my travelling companion and I we spoke 7 languages but not a word of Korean. Even on the plane and in the airport where I am sure english is more common, I found it extremely difficult to understand people and be understood.