depends on which country you live in, in Canada La Chateau is a great one, they have some beautiful dresses. There might be a la Chateau in U.S.A. too, but if not you can shop on their website and have it shipped to the U.S.A.
So many places .. Are you looking for a short dress?: try Goldmans, Rue21, Forever21 maybe, goodwill has awesome long dresses and short ones as well!! If you heard of goldmans before go there ... You can get coupons also type up on google: printable goldmans coupons!! Good luck finding one!!!! :D