Luckily, I happened upon FlowerGirlDressForLess.com. We found an adorable, simple, white satin dress for about $40 (Style # 5287W) that we paired with sashes that matched my bridesmaid dresses. [Check out the website for gobs of other affordable dresses!] The result was fabulous and super-cost-effective!
Here's a pic of my cute little flower girls (and ring bearer!!):