It's a real PIA to alloy pure antimony into pure lead. Antimony melts at a much higher temperature, like 1167 degrees - pretty ferocious, if even possible with most hobbyist lead melting gear. You melt that stuff first and then add the lead. Once it's in solution then you can resort to "normal" melting/casting protocols. Guys I've known who've done it pissed and moaned and swore "never again". You'll still have to add a little tin to promote mold fillout anyway probably. Printing type alloys all contain a small percentage of tin in addition to antimony (and arsenic, but we won't go there).
Depending on your end needs of course, a simple tin/lead alloy is simple, easy, and can cover about 90% of all bullet hardness needs, rifle and pistol. Example: if simple handgun bullets are in the offing 1:10 tin:lead is sufficient some pretty hot loads (Elmer Keith espoused it), 1:20 good for warm loads, and 1:30 is plenty hard for low velocity target loads (check the hardness of factory .38 wadcutters and get back to me).
If you're hellbent on super hard bullets (very little actual justification, that), then do as suggested and snag some monotype or linotype and go from there.
