Forever 21 is a good start. Most, if not all, of their dresses, are under $30.
You could also check out Delia's, Victoria's Secret, and department stores like Kohl's and Macy's. Kohl's is nice because they have a lot of the same brands as other stores but they have a sale 99% of the time. Target has really nice designers doing clothing lines for the store, too.
The second one has dresses that are all under $50. If you look on the lefthand side of the screen, you'll see tabs for under $100, under $150, and so forth. They've got a cute little black dress on there:
Surely you'll be able to find one on there that you like. They should work as long as you're just going to the wedding and not actually in it. Some are a bit too much, but you'll be able to pick those out :p
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