There many places that one can obtain lead for casting. Probably the best place to go is your local salvage yard. Ask for lead gaskets, wire sheathing, lead pipe or flanges.
Another option is to order online through Roto Metals. Use promo code LEE19 to save 10% on your first order.
Casting for black powder bullets, round & maxi balls, shotgun slugs and similar projectiles should be made from pure lead. Wheel weights and other lead sources that have tin or linotype in it make good bullets for pistol and rifle projectiles, due to the hardness that is associated. Sources such as battery lead or shot should not be used, they contain arsenic and are dangerous to work with.