I think you'll have a hard time finding roundballs bigger than the size for a Brown Bess (10 Gauge). Dixie Gun Works or the Log Cabin Shop might have 10 and 12 Ga roundballs. Unless you have a little cannon, you won't find the ballistics too appealing. 12 Gauge roundballs in a double barrel would make an interesting back-up gun for those with steel nerves and a gun with good ignition qualities. I've shot some in a BP double barrel, and up close, they are a force to be reckoned with.
In the African Ivory hunting days, a 4-Bore was quite common. I remember a biography of one such Ivory hunter, 'Col. Something or another', who was well regarded for his courage and marksmanship. He noted that he seriously regretted having fired the 4-bores, for he developed significant flinch that took him some time to overcome. Such rifles were solidly built, heavy, stout, and often loaded with charges that approached 1 to 1 weight of ball to powder. If you consider the weight in grains of a 4-bore (4 balls to the pound), it doesn't seem safe in this day and age. They would approach an elephant on foot, leading a horse, shoot a large bull, the mount the horse and gain some distance while reloading.
Personally, beyond .62 Caliber or so, it takes a LOT of powder to generate enough velocity to have a decent MPBR.
For lead, try hospital x-ray rooms for shielding, sources for lead pipe (plumber supply), and some lead sheeting is used for roofs. (Also underwater cable shielding). Soft lead is much more important for a muzzleloader, so the round ball obturates. Wheel weights are too hard.
Dixie Gun works will make molds to a specified size, but they are rather crude.
On the positive side of things, a big soft heavy lead projectile moving at moderate speeds has power that numbers alone do adequately portray.
ice...
