I bought Disney Christmas cards at two stores in WDW. One was at the scrapbooking store in DTD, it's called Disney's Wonderful World of Memories.
The other was in Hollywood Studios, in 2006. It was the store right before the entrance to the Osbourne spectacle of lights. I was waiting in a mob of people to enter, and I decided to go into a store and wait for the mob to dissolve. It was just a regular gift shop, but they had really great holiday cards.