You used to be able to buy lead shot at gun shops. Don't know if they still sell it. They might also have steel shot, which would do the same thing. Sand in zip lok bags might also work. If not, you can get lead at a plumbing supply house, or a scrap yard would probably sell you some cheap. Or you can go to a tire shop and buy some of the tire weights they take off and melt them down into what size you need. I have lead ingots in about 5# increments, and also some lead "muffins" that weigh about 2-3# each. ( I used a muffin pan for a mold.) If you go the route of melting lead, keep in mind the fumes are very toxic and there is some risk of working with melted lead.
Randy