Another thing to look for at the thrift store are leg and ankle weights in the sports section. I get quite a bit of lead [as mentioned already] at the gun range.
For those who don't know already, it is not a good idea to add jacketed bullets from a range into a pot of molten lead because there could be moisture in them, and anything that I put into molten lead [eg. ladle, slag skimmer or more lead] I heat next to the pot to drive out any moisture. Water and molten metal of any kind is a bad combination.