The range scrap from my local range is about BHN 10 so pretty good up to about 1500fps with most calibres. I expect most ranges where people shoot cast will be similar.

I've bought tin and antimony from Carn Metals. I'm not sure anyone else is selling antimony. Melting pure antimony into an alloy is really slow. https://www.carnmetals.co.uk/

Modern pewter is mostly tin, so is lead free solder. If you can find any old tin-lead solder alloy, it has a lot of tin in it.

BB