Curtain Call Collectibles has Mickey ears and hats... turn right right after you pass under the train station entering the Magic Kingdom. You can also get hats/ears embroidered here.
Or start in the Emporium, and continue through the shops on that side of the street until you hit Casey's at the end. There are a couple of shops in there that have a good selection of ears, baseball caps and the like.
The shop at the exit of Mickey's Philharmagic usually has a decent selection of ears and hats, as well.