I know that you can get stamps from the Greetings from California store inside of DCA. They are in a vending machine near the door at the closest in-store point to the main entrance of the park, if that makes sense. They come in packs of 4 or 6 or 8 or something and I have no idea if they cost more or not - I am from Canada so have no idea what normal postage rates are, but they do sell domestic USA stamps as well as ones to Canada and International destinations.