Lead, regardless of whether it's diving weights or X-ray shielding or stock car bricks, becomes "pourable" once it reaches its melting point.
If you're looking for something easy to find, buy 25lb. bags of shotgun shot from your friendly local gunshop. You'll just have to find something to melt it in, and then a mold to 'pour' it into.
Better advice: Find a local scrap yard, or a machine shop. Buy some steel plates, or go with barbell weights, as have been already recommended. Much more sturdy. Much easier to find. No concerns about toxic heavy metals.
And, as all experienced racers know, iron is much faster than lead, pound-for-pound.