Watch out for the Zinc weights now. My observation is that about 50% of the weights in my local tire store's scrap are now zinc. It takes only a very small amount of Zinc to wreck the castability of a LOT of Lead.
Some useful threads over on CastBoolits:
http://castboolits.gunloads.com/show...ad.php?t=85705
You will also find steel weights.