we got our's (in 05) in a shop on Main Street in the MK.
according to the UG book:
The Chapeau on Main Street
we bought ours the very end of the day when the park crowds we're leaving after Wishes...I think it was at the very end of the street toward the entrance, just a little shop. But the CM's working in there were FAST!
We got the classic black ears for our DD an DS twins then 6 and the pink (minnie) ears for our DD then 2 1/2
Our kids now 9, and 5 keep them up on a special shelf w/ other Disney things and have often worn them to school on "hat day"
One of the best quintessential (IMHO) souviners you can buy. Just don't forget to take a picture of your kids weraing them in the park.