To be extremely general, the foam is neoprene. However, as all things are in racing, there are a ton of different variables that are played with to create all the different versions and specifications that we actually use in racing. Some compounds contain natural rubber, others contain all man-made. All these factors make a difference in how the tires work.
Knock yourself out making your own tires, however, expect to spend large sums of cash, and be prepared to test your patients working out compounds that will actually work.
Speed Merchant Rev7, Tekin, TQ Racing (wire), Team Tamale
RC Excitement - Buy where you race, support your local tracks.