As the PP said, you can find an assortment of clothing all around the park. The best selections are probably at the Emporium on Main Street in DL and Greetings from CA in DCA. I would also agree that things can get pretty pricey in the parks. If you are looking for something cheaper, there are always tons of little shops in the parking lots of every hotel on Harbor that might have something at a better price. Unfortunately, quality might be less as well. I bought a sweatshirt in DLR almost 5 years ago, and it still looks great!
And if you plan on getting something, evaluate the size or even try it on before buying, because I have noticed that the adult sizes tend to run just a little small.