When I buy it, I use Rotometals.com. They sell 99% pure lead in all manner of shapes and sizes. Last time I bought some it was something like $99 for either a full or half pig, I can't remember. They had a thing running then that if you allowed them to ship it to you cut in half, the shipping would be free. I said what the heck, go for it. I don't care what form the lead comes to me in, its all gonna become round balls anyway.
Another place I get lead from is the gun club I belong to. After a shooting session, if no one else is shooting of course, I scrounge lead from behind the back boards. You have to fish for it though as not all of what is there is usable. A lot of guys shooting slugs, sabots, etc from modern guns at this range and that stuff is too hard for what I want. But if it scratches with my finger nail, it goes into the container. I usually come home with the same or more lead than I started out with that day.