I disagree with HH on his comment about pureness. You want as pure as you can get for your purpose. Look for a local salvage yard for lead. They often get old x-ray shielding, pipe and electrical casing, etc.
Finding it in handy ingots will be a problem. The people who have ingots have gone to the work of scrounging scrap, melting, cleaning and recasting into ingots for their own use.
Finding lead is an ongoing endeavor. My last came my way more than five years ago. I'm now down to under 100 lbs on hand which means time to start scrounging again. I'll probably go to the local metals salvage yard. He had it last year at 50 cents a pound.