Medical lead is about the purest soft lead you will find. But you don’t need lead that pure for blackpowder shooting, regular unalloyed lead will work just fine.
Cast bullets have alloys to harden the bullets. You don’t need that. Blackpowder guns operate at such low pressures that hardened bullets won’t upset to fill the grooves. The lead in cores of FMJ’s is dead soft lead, which is great for blackpowder.
Unalloyed lead is used in various applications. This scrap lead, got at a scrap yard, was dead soft.