Roof jacks, lead sheet flashing, and lead shower pan liners are good scrap sources for round balls. The material is not pure but it's close. The balls will get slightly harder as they age a few weeks from when they were first cast, but still good.
Wet-cell automotive batteries will kill you, not kidding. The problem is calcium added to the plate material to make them hard and vibration resistant makes phosgene gas when heated in the melting pot and one whiff is all it takes. Also, the junk you skim off will release extremely toxic gas when it absorbs moisture from the air. Bad, bad stuff altogether.
For a smoothbore, the lead alloy wheelweights work fine for round balls only. Save your soft lead for rifles, revolvers, and those funny looking pointed things with hollow bases that have to be soft to work right. The zinc alloy wheelweights are no good, sort them out and recycle them.
Diving weights are usually whatever is cheap from the scrap yard and are often contaminated with zinc wheelweights that were indiscriminately melted in with the other scrap. Zinc, even in sub-percent quantities, makes lead and lead/ antimony/ tin alloys very difficult to cast.