I have purchased all of my high school dresses from E&E. They always gave me a great price and even searched for dresses they didn't stock. The ladies there are always extremely nice and they are honest, which to some people might seem counterproductive, but you don't want to look like a tramp.
Pros: You can order any dress, if you see it online or in another store, they will find it!
Cons: there is only one store, and it's 200 miles from me :(