It's great to have a campfire when you are camping in Algonquin. And buying firewood is no problem at all here. They sell it at every entrance gate to the campgrounds. You have to watch the opening hours though, I don't think you can buy wood all day long. But I mostly pick it up at the end of the afternoon and haven't had any problems at all.
Next to the little grocery store at the 'Two Rivers Campground' is also a little souvenirshop. You can buy anything here from Sweaters, T-shirts, books, and all kinds of other souvenirs. And of course you can buy your postcards that you want to send to your VT-friends here as well :-)
Another option to buy souvenirs is to go to the Visitor Centre. In there they have a little shop with lots of great books about Algonquin Park and also a great selection of nature books. They have some souvenirs as well and of course postcards.
There nearest shops outside the park are very far away, so it's best you bring all the supplies you need. There is a little grocery store in Algonquin at the "Two Rivers Campground", where you can buy some basic groceries. It's great when you are for a few days in the park and you forgot to bring something or need something fresh like bread, fruit or meat and something I always seem to run out of : bottled water. But don't count on a huge supermarket with everything you need, it's very basic, mainly things for a quick meal.