Honestly, I can't remember the last time I was at a pet store... everything that I buy for my rats either comes from fabric/craft stores, grocery stores, Walmart (for Tupperware containers and other such things...)
You'll learn how to think like a rat after a while. You'll walk around a store and the whole time you'll be thinking, "Oooh! That could be a bed! I could play with that! That would be SO MUCH FUN!"