I've always had a lot of luck with
www.bluefly.com especially for wedding related events.
You can search by size, color, designer, price range, day/evening, long/short. The quality has always been exceptional and I have seldom needed to return anything because of fit.
Many of the dresses are an extra 20% off for a pre-spring sale -
http://www.bluefly.com/pages/browse/...ptFolderId=933