Like SleepingBeautyDreamer said, HD is a great place to get precut, any color (although we are partial to LIMEgreen, aka Alien Green)). On the thread she mentions, I think post #3232 (a condensed version of the rather long thread) there are even instructions on how to get the Mickey head off the backing of the paint chip. The Mickey heads are about the size of a half dollar. Free, precut, and any color you could want.
I also have a couple of different sizes of punches of a mickey head that I got from A.C. Moore, a craft store. The largest is about the size of a nickel, the smallest is about the size of a dime, including the ears. They have these at Michaels, too, and I have seen them in the scrapbooking sections in a couple of stores at WDW.