This is a listing of some of the classier and modest prom dresses available. Mostly, it appears, from LDS suppliers. The LDS should not be the only conservatives with class.
http://www.adressyoccasion.com/Bridal
http://margenesbridal.biz/view-wedding-gowns.php
http://margenesbridal.biz/view-prom-dresses.php
http://www.latterdaybride.com/modest-prom-dresses.php
http://www.latterdaybride.com/modest-wedding-dresses.php
http://www.beautifullymodest.com/categories/Modest-Wedding/
http://www.unique-vintage.com/shop-prom-dresses-style-modest-prom-dresses-c-505_525_533.html
“Dressed in White”also has nice stuff. I bought my daughter’s first Communion dress from them. Yes, they’re LDS, but unabashed about marketing to Catholics.