I got my niece's face painted at AK in Dinoland near the flying Dinosaur ride, there is also a place in Africa's Hambre village that does it and I saw a face painting stand at MK right past the enterance to Toontown on the right hand side.
It costs $10-$15 depending on the design you pick. Of course my niece picked one of the $15 designs, but it was beautiful, she LOVED it and so many people told her how pretty it was, it was worth it!
It took about 10-15 minutes to do. It's amazing to watch them do it too.
One bit of advice. Get it done after you have done any water rides like Kali River or Splash Mountain because it will smear with alot of water.