Most divers have lots of pure lead weights laying around.
Most divers quit diving in just a few years, so there's plenty of this stuff available.
The problem with it is the water that might be in it.
If it isn't removed it will turn to steam when the lead is melted and make a real mess.
Heating the lead, slowly, and letting the water escape safely should make all this unwanted lead ok to use.
Lock the doors, they're coming in the windows.