Depending on the age of the little girls, you may soon be able to find good deals on dresses in nearly all department stores as they start carrying white & ivory First Communion dresses. (First Communion is a sacrament traditionally received in the Catholic Church around second grade.) My flower girls are both 6, so this will be a good money-saving option for us. And if you live in an area with a sizable Catholic population (not Oklahoma, where I am!), you can also check in the phone book for Catholic bookstores--larger ones sometimes offer FIrst Communion garb.
"Do not fear, for I have redeemed you; I have called you by name; you are Mine!" (Isaiah 43:1)