You can get them pretty much anywhere they sell hats! Most of the resort gift shops have them as well.
And, there are new ears you can "customize" as well. (I'll see if I can find the post on that and link it here).
ETA: here's the link: http://www.disboards.com/showthread.php?t=1766207
The new customize-able ears can sometimes be better for the guys since that top hat seems small/tight on many men.