No company I know of makes them now however many re-enactors make their own to duplicate the cartridges that were used back in the day.
There have been several posts about how to make these on the forum.
They require a wooden dowel the same size (or close to it) as the ball, some paper, string, powder, ball and glue.
The "cartridges" used in muskets or rifles are usually torn open with the powder dumped down the bore followed by the ball, sometimes still in the paper wrap and sometimes not.
With Cap and Ball pistols the paper was often treated with Potassium Nitrate (Salt Peter) so it would be totally burned when the cartridge was fired. Some modern folks use cigarette paper to make this style cartridge.
These Nitrated cartridges were usually placed into the chamber with the ball outermost. The ball was then seated below the chamber mouth with the loading lever.