I just bought a Santa hat with Mickey ears at Five Below for $5 (Nov 19). Thanks everyone for saying where they are sold.
Note that the Mickey ears are soft fabric. I guess I expected them to be hard plastic.
Also, the quality of the ones here wasn't great. When you pick one up, look carefully; you might find that one is coming apart more than others.
While you're there, they also have T-shirts for various tv shows/movies (Rudolph, the Grinch) as well as Disney-themed. They also have Christmas wrapping paper for tv shows/movies, but you might find that elsewhere for less money.