I recently visited the local scrap yard and picked up what I found to be some sort of lead wire coating. I'd never heard of it before but it turned out to be the easiest thing in the world to melt down. Nothing but some oil residue on it in terms of dirt. It bends easily so getting it in the pot was no problem. Some research on here says it's likely as close to pure as one could find and some rough weight comparison of ingots has verified that. At a dollar a pound it's not cheap but I'll take it.
I also got some lead sheeting that looked like it came out of showers or maybe off of roofs. This wasn't bad either but had quite a bit of dirt on it.
A sailboat keel would be great, and fairly likely for me to find in my area but I'm not sure it would be worth cutting it up to melt down rather than just take advantage of these more convenient forms.
What is your favorite form of lead to find?