If you don't mind buying online check out newport-news.com, they are relatively inexpensive and always have tons of dresses. If you want to go out shopping check out Old Navy, Target and JCPenny or if you're a true bargain hunter like me go search through stores like Burlington Coat Factory, Marshall's, TJ Maxx, Ross or Last Chance, all of these stores are hit or miss but you can get some really great deals if you hit!
Edit: Here's a cute one from Newport News, it's $39, comes in sizes up to 18 and the big bow sash is removable (which is what I would do!)
LINK!