It seems a bit odd to call designer outfits "costumes," but once they have been used on film that is what they become. There are several pieces from the movie The Devil Wears Prada up for auction on eBay to benefit the nonprofit group "Dress for Success," which helps economically disadvantaged women start on the road to success by preparing them for interviews and other career opportunities. This Donna Karan two-piece crepe suit was worn by Meryl Streep/"Miranda" in the film. No size is indicated and it comes with a certificate of authenticity. The current bid price is over $500, but the auction doesn't close until July 3rd, so there is plenty of time for that number to go higher. And, for a good cause, why not?
An alternative source for movie merchandise is the Devil Wears Prada boutique that Deidre mentioned a few days ago.