The Lyman No 2 alloy is a classic. It's 90% lead, 5% tin and 5% antimony.

Tin is relatively easy, e.g. solder of known composition e.g. 60/40.

Antimony is harder, however this company claims to sell it (it has a highish melting point, so see if you can get it in alloy with lead):

http://www.carnmetals.co.uk/

You didn't want a scrap solution, but wheel weights used to have ~5% antimony and a bit of tin. Don't know what they are made from now, but maybe worth investigating.

Another scrap source is of course lead from a range.