The adult costumes are usually up by the end of August, but I recommend waiting as the closer you are to Halloween, the cheaper they get! (Although it's a balancing act...wait too long and the size you want is sold out.)
The stores don't usually get adult costumes, but the website usually gets a small collection. Some years have had better selections than others. They usually have at least Minnie Mouse and Tinker Bell. I have bought several costumes from them over the years.
Also, buycostumes.com has a fair number of adult sized Disney costumes available year round, although they aren't often as nice.