Here are two other sources for learning sponge ball magic.
1. Ear Plugs by Jay Sankey. (Instead of sponge BALLS he uses giant sponge ear plugs about the size of sponge balls. This could be a plus because the audience would connect with them better than a random object which they have never seen before). It comes with a full length DVD to teach you sleights and routines.
2. Buy a set of Goshman sponges. Each set comes with four which is good for many routines. Inside each pack there is a peice of paper with a full routine on it. Learn that, and then play around with it to make it your own.