Khornet stated he is looking for "PURE" lead for a muzzleloader, not harder lead alloys for modern firearms.
Khornet, I have found pure lead from a number of sources. Bought 400 pounds from a hospital in west Texas. The lead resembled a jug and had a lead cap to fit over the hole where an element called "technesium", if I remember correctly, was stored. Made wonderful .490" balls for my .50 Mountain Rifle.
A telephone repairman gave me sheet lead (~3/8 or 1/2" thick X various lengths/widths. Also have used lead water & sewer pipes with success. Generally, wheelweights contain too much tin, antimony and arsenic to make soft muzzleloader balls.