They do sell it in the MK just near the entrance/exit on Main Street BUT not sure its the stuff thats spun in front of you, but it does come in big bags and its very nice indeed!!!
I am so glad you asked this question! My 4 year old daughter is a cotton candy fanatic! She is saving her money to buy cotton candy at WDW when we go this fall but I didn't know if they had it there either.
The candy shop at MK has cotton candy and I have seen the cast member spinning it. Also, the Villians Shop at DHS has cotton candy, candy apples and an assortment or different treats and they spin it there also.
You can watch them spin the cotton candy right there at the Confectionary at MK. They bag it right away and you can buy it fresh! We love cotton candy, and always say we're going to buy it, but then forget every time!