You're sending a business holiday letter or something? Is that why you need the holiday paper? Regardless, Kinkos always has some paper running around. So does Michaels and Hobby Lobby and places like that. I would say Target but I'm boycotting Target right now (see Current Events post) and I'm not even sure they would have it.
I'd go to a craft store. They'd probably have the best selection.
Maybe an Office Depot or some such store would have holiday paper now?