I don't think I've ever seen so many Audrey Hepburn dresses up for auction in such a short period of time! Just this morning it was announced that Christie's would have a film memorabilia sale at its auction house in South Kensington on December 19th, and a dress made for and worn by Audrey in Charade will be up for sale. Unfortunately, Christie's website doesn't have any details about this auction up yet, so I can't find out which dress it is. I'll keep checking, though, and let you know as soon as I find any information.
In the meantime, you can read more about the auction as a whole here, and see some other great pieces that will be up for sale that day.