It took some time, but there is a new dress from Summer available on ebay. Its a dress she wore on The Cape that is auctioned off. It doesn’t look like that the bailey dress is from a particular episode of the show. Looks like it was used for promotion. The auction will end in about 2 days. Unfortunately there are no more informations about it available.
Update: The item is back online. This time the auction will end in 4 days and more infos are available. According to the auction details its from episode 3 Kozmo.