If you catch them remodeling a hospital or doctors office, the x-ray room has lead sheeting in the walls. It is soft enough to fold up like thick cloth. You can buy a truck load cheap if you make the right arraingements. The containers the products nuclear medicine uses for radiation treatments are shipped in molded plastic containers with lead inserts. Most of them are too hard to use with muzzleloading, but a few have 14 pound pure lead containers inside them. Take out a couple of screws and the lead is free. Hospitals tend to collect these because it costs to get rid of them. A person that will remove the lead, trash the plastic, and remove the lead from the premises will often get permission. You have to take it all, so you need a friend or smith the does hardcast to take the harder material. My present supply is lead pipe, lead sheet from an x-ray room, and one 37 pound medicine container melted, mixed, and cast as ingots.
The local smith sells virgin ingots for 20 cents a pound I think in 10 pound blocks. Shipyards, for those of you on the coast or near the larger bodies of water, use larger amounts of lead in keels on the larger boats. Since they buy large amounts, they might be a cheap source.