The lead from RMR is just about perfect for casting pistol boolits. You just need to add some tin and you're good to go. At about $1.50 per pound it is probably the best price you can find online right now.
The RMR alloy is 2.5 to 3% antimony, so adding about 1 oz of tin every 6 lbs will get you an alloy with 1% tin. Start there and adjust up or down as needed.
As for tin sources, you can try to find pewter at thrift stores, plumber's or electrical solder, look in the swapping & selling or vendor sponsor sections, or go to rotometals. It's pricey, but a little goes a long way. Even if you buy just a pound or two from rotometals, it will only add $0.25 to $0.30 per pound to your alloy cost. Each pound of tin will do almost 100 lbs of alloy.
https://www.rotometals.com/tin-cut-w...d-made-in-usa/
https://www.rmrbullets.com/shop/rmr-...nnel-hardness/