He said that he uses pistol bullets, so I guess you have to find the caliber that coincides with the guage, then find the sabot sleeve that will work. I'm not sure if you can use the components from muzzle loaders or what...so I'm not much help there.
But, it should be something like this:
BPI BLS-12 Ballistic Sabot 12 Gauge with...m Package of 20
