Nobody really sells narrow caliber BRI/Winchester type sabot slugs and sabots for reloading that I know of. Most slugs are either traditional rifled slugs, or some variant of a Brenneke style that is designed to be shot in a modified wad section. These usually run somewhere close to 12gauge bore size, not closer to .50 as with the BRI/Winchester style. The Lyman is another larger caliber slug, though it has a wasp waist.
Muzzleloaders would be interesting, but the only ones with a bore close to a 12gauge would be a Brown Bess or Charleville musket.