Straight up wheel weights will get the job done. Visit your local tire and mounting shop. Many times, they're more than happy to give them to you for free. Just take them a dozen donuts the next morning. Friends for life.
Melt the wheel weights at a temperature that melts lead and nothing else. The steel clips will float to the surface. The rock hard zinc weights will float to the surface. Skim them off and you have really good lead for bullets and balls. No need for additional antimony. Toss in 4% tin for mold fill out and you'll be fine.
To save a lot of money on tin, go to your local Good Will store. Buy up all the old pewter laying around. Melt it down and you'll have high quality tin. Most pewter will say PEWTER on the bottom of the piece. If it doesn't say pewter, don't buy it. It's not worth the risk.
Sailboat ballast is another great source of lead. Get a big piece of that and have a small test piece sent to Roto Metals. The'll test it and tell you exactly what's in it. Sometimes you'll get a high antimony content.
Good luck.