ebay, charity stores, your mums closet from the 70s
You can find vintage dresses for sale at garage sales, stores where they are not exactly vintage but are designed with vintage designs such as etsy.com or urbanoutfitters.com.
Modcloth.com is know for vintage items. Search under dresses, and they may even have a bridesmaids section. You can find affordable dresses there that even the bridesmaids will love.
Vintage markets, internet, charity shops, family members?!
Try online at Ebay.com. You can also find cute clothing on jimmyjazz.com, oldnavy.com and thechildrensplace.com. You should be able to find great dresses here.
Vintage clothing was normally manufactured with better material and fabric than what is used now. You can try www.ebay.com to find some good cheap vintage outfits, and dresses.
walmart
You can find vintage dresses online at Amazon and eBay. If you purchase a dress on Amazon using their Amazon Prime program, then you will receive free shipping and will not have to pay any sales tax.
At the related link below you can find dresses online and free shipping.
boohoo.com cute dresses for very cheap prices :)
You can vintage dresses off this website (http://www.modcloth.com/) This website has all sorts of dresses (modern all the way to vintage) so you shouldn't have any problem with looking for a vintage dress.
J.C Penny
There are so many places to find cute dresses, both online or in stores. Walmart.com, Craigslist.com, are two good online options, as well as Ross, TJ Maxx, and thrift stores.