I think the best dresses are ones that are handmade according to your pups measurements- they always fit the best- I obviously can't resist some cute and cheap PETCO or WalMart ones every now and then but they don't alwats fit perfect.
There are so many vendors that sell great dresses here- your big problem will be finding the money to buy all the dresses you want.
Here are some vendors you can start checking out- hopefully others will jump in with other recommendations because I can't remember everyone-
FuFu Fashions custom dog clothes and apparel Prissy Paw Fashions Home Page
YT member 107barney-
Washable and reusable dog and puppy pee pads, belly bands, and diapers at T. Bumpkins & Co.
YT member OwnedbyFiona-
Fairy Tails Fashion