http://www.ballisticproducts.com/
http://www.ballisticproducts.com/RSS-12-Rigid-Structure-Sabot-only-12ga-50_bag/productinfo/322RSS/
The first link is to the BP site, the second is to their new sabot. I say new because their first attempt was a failure. It had a collapsible cushion section in it, like a target plastic wad. It failed to stand up to the pressures generated by sabot slug loads.
I spent a lot of money and a lot of time trying to get that sabot load to shoot. Pretty close to a grand! (New rifled barrel, shotgun scope, drill press and hull vise for roll crimping, components------,well you get the picture.
As for the lee slug
MOLD, it is made to fit inside a normal trap/target wad. You have to be set-up to cast lead boolits, then you can make the key drive slug.
It's called the key drive because it has a rib across the bottom of the hollow base to grip the plastic wad that's carrying it. It will work in smooth bore shottys, and behaves like a sabot in rifled barrels. I have both molds, 7/8 and 1 ounce versions. I plan on spending some time this summer to see which one is best.
As for driving a round ball down a rifled barrel, I would advise against that. You would experience horrible leading. So bad that you would NOT see any accuracy. Also the composite wads needed for a round ball load are available at BP, look around at the first link above. You'd need some sort of over powder wad, and some fiber cushion wads. If some way could be found to lubricate the ball, you might get away with it.