While hunting around to buy three distinct bouncy balls sizes in bulk, I found this link on making your own bouncy balls. It may surprise some, but I am not crazy enough to make my own since I'll need 100s. But I could see making bouncy balls sometime with my cousins.
(Update: Feb. 2010. I followed the above recipe with my cousins and the bouncy balls didnt bounce very high and they cracked on the surface but were gooey in the middle. They did however bounce pretty good on the trampoline.)