Clearballs.com has many different ones. Then there is also the
Roto Grip Spare Tire, which is discontinued but can be found on e-Bay and probably a few other sites. The Sugar Ray (boxing
glove) is also discontinued and can be found on a few sites, the same for the
Track "T." Lane #1 has a clear ball with their Diamond Core in it. There are a couple of different Skulls, one is by
Ebonite, but I prefer the one by
Elite.