One must develop the fine art of scrounging to get good lead.
Admittedly, it is not always easy.
Around where I live the metal salvage yards are an excellent source. Try to get roofing, x-ray, electric cable wrap, pipe lead and others. Do the fingernail test on everything. Roofing and x-ray are probably the best types to work with and are consistently soft and pure.
Buy as much as you can at a time, it is not getting easier to find and certainly not getting cheaper.
I am down to about 100 pounds and will have to start looking for more. I have inquired at a salvage yard, he has quite a bit he'll sell for 50 cents a pound.