I personally use the local pet stores that sell various type of things, such as food and toys. They're really good and the salesmen know a lot about pets and are really helpful. The only thing that bugs me about those kind of stores is that they're usually really expensive and can make your wallet appear pretty empty in a short period of time.
However, if I don't feel like dragging myself into those pet stores I just walk to the nearest supermarket and go to the pet-side. They usually just don't have the brands that I buy for my dog but I've remembered all the places that do. Usually supermarkets are noticeably cheaper, but you don't get any service so you need to know exactly what you're doing. I also do some internet shopping, but there's the same problem with that and the supermarket: you need to know everything by yourself.
