Oooh... now I wonder if I can find one of those toys on eBay!
LOL...they're all over eBay...because people are just reselling from various Toy Stores or wherever they are getting them in bulk. Prices on ebay are fairly close to what you can get them for in some of the discount stores. Toysrus has them now for $14.99 and they're going for at least that much on eBay.
I remembered seeing this project in my news feed a couple of days ago and went back to look at it again today after seeing the movie. I think I'm going to give this a try for my first Netduino project. Too bad that ring around the outside is blue plastic, I think you could modify the circuit to allow for two different colors (blue and red/orange would be my choice).