NAA sells them: https://northamericanarms.com/parts/parts-cb/
They are 30 grain lead balls
I think doing a RN mold if you can find one is the way to go IMO. You could possibly create a 40 grain projectile to up the performance over the round ones. I believe they are right around .221-.222 in diameter but you would probably need to mic the chambers and bore if the specs aren't available.