The best DVD on spongeball that I learned the most from is Jay Noblezada's DVD and Sankey's Earplugs DVD. The latter is a new look on sponge balls that every people can all relate to.
Also, while this might not relate to the main point of this thread entirely, I strongly advise you to take a look at an effect called "The Okito Voodoo Doll" This trick is perfect for every audience and situation and is a reputation maker. Please do check it out, you will not regret it.