There is a big supermarket on Amazonas (between Guayaquil and Salcedo) where you can buy basically everything. I came here several times while in Latacunga, but when I was going to buy supplies for the Cotopaxi climb I couldn’t find energy bars or a good mix of nuts that I wanted. I remember seeing another big supermarket on the other side of the river, near Avenida 5 de Junio, so I walked there instead. At this supermarket I found a big variety of granola bars and a bag with mixed nuts and dried fruits that was perfect to bring to Cotopaxi. While I was there I also bought batteries for my head torch, chocolate and Gatorade to drink on Cotopaxi.
Update 2012: The supermarket on Amazonas is called Santa Maria. Also this year I found good energy bars here that I bought to bring on hikes.
Latacunga's market is one of Ecuador's most authentic. In fact, there are few tourists shopping here. It's colorful and blessed with a nice setting amongst the provinces stunning mountains. It's a place where locals come to buy their necessities as well as have something to eat.
What to buy: Don't expect a lot of fancy crafts or typical tourist ponchos. They sell what they know locals will buy so you'll see plenty of jeans and CD/DVDs. There is also a huge selection of produce.
What to pay: Be careful what you choose fruit-wise as things out of season or not grown locally can be more expensive than you planned. We bought some delicious if somewhat expensive cherimoyas.