Although a wedding is a fairly intimate moment shared with your closest family and friends, it can also be a time to share your joy and good fortune with those less fortunate. Yes, you can have the dress of your dreams while being charitable (isn't the Universe a wondrous thing?).
Charity - A truly unique shop is The Bridal Garden in New York. Imagine owning a designer gown at 70% off the regular price (think Vera Wang for $895)! And imagine all the proceeds helping disadvantaged children. The line-up of designers who graciously donate their frocks reads like a who's-who of the wedding world, with names like Badgley Mishka, Amsale, Lazaro, Monique Lhuillier, Reem Acra…and the list goes on.
If you are in the New York area, definitely check it out and if you're not, it could be worthwhile making a trip to the Big Apple just to see if The Bridal Garden has 'the' dress for you. As all their stock is donated (including veils and headpieces), the store's wares are constantly changing. Be sure to call ahead to 212-252-0661 as The Bridal Garden operates by appointment only.
Another charity option is the Making Memories Breast CancerFoundation which offers new or gently used dresses via shows throughout the USA. All sales benefit women suffering from metastastic breast cancer (Stage 4, usually considered to be terminal), allowing them to have a unique trip or experience with their families. Check their website for the show and date at a town near you.