lead
Just some food for thought here. Don't know where you're located but, do you have any local shooting clubs?
Better still, any blackpowder shooters. Those guys go thru a ton of lead (play on words), they have to get it from somewhere. Most, not all mind you, cast their own bullets.
Also depends on how much you need. If it's a large amount, a vendor like Tod showed you is your best bet. If it's just a couple bags, someone that handles shooting supplies might be the ticket.
Good luck,