If you graduated from high school in a white dress, that one is likely appropriate. If not, there are a ton on sale in department stores now.
All of these would be appropriate for my org and chapter, and seem similar to what my friends in other orgs have worn, though I actually don't know their regulations:
http://www.lillypulitzer.com/The-Lit...ttlewhitedress (if you have a store near you, there are a lot of white dresses on sale at Lilly now- a few for $99 or $114)
http://www.dillards.com/webapp/wcs/s...924&R=03302112
http://www.dillards.com/webapp/wcs/s...924&R=03325867
http://shop.nordstrom.com/S/3103984/0~2376776~2374327~2374331~6014133?mediumthumbnail= Y&origin=category&searchtype=&pbo=6014133&P=1
http://shop.nordstrom.com/S/3085563/0~2376776~2374327~2374331~6014133?mediumthumbnail= Y&origin=category&searchtype=&pbo=6014133&P=1
sorry if this was a little excessive, I really want a new white dress...