I use Hornady 000 buckshot in my 36 cal. firearms, it is .350 diameter and comes in a 5 lb box.
Not sure what your best bet is for supply.
Just to be clear though, a .36 muzzleloader uses a .350" ball but a .36 cap and ball revolver uses a .375" ball.
Great idea.
Do you get the swaging when seating in a BP pistol cylinder ?