My preference for PRBs in a rifled barrel are less than 10bhn, and preferrably less than 8bhn. I recall reading about the white hunters in Africa using "hardened balls", but they would have close to that 10bhn. Softer compresses, loads and expands more easily.
Harder balls can be loaded in smoothbores and will still expand.
If you're scrounging, look for old lead pipe, cable sheathing, roof flashing and other sheet lead. Stick-on wheel weights are also good.
Modern clip-on wheel weights can be lead based, iron, or zinc. You can't melt the iron, and the zinc will contaminate lead, so avoid those.