kadima, lot of good advice given. Some got it right for wrong reasons, but what the hey.
Stay with pure soft lead for a variety of reasons, some given others not.
Someone gave best advice for finding lead, 'scrounging'. Keep yer eyes open for opportunties. I get old electric cable insulation (?) from scrap yards, x-ray shielding and whatever else I can find. And, being a die-hard, long time traditional muzzle loader, I am very fussy about getting really soft, pure as possible, lead.