Use the simplest method available to you...
The best advice has already been given- obtain some typemetal or get Rotometal's "superhard" alloy.
For purposes of economy, the purchase of pure antimony should be considered only when already-alloyed sources are unavailable.
Or if you absolutely "have to know" what's in your alloy. Considering the fact that your base metal composition is not 100% certain (scrap sash weights), you are not operating under precise parameters to begin with, so this would not apply.
Do this stuff long enough and eventually you may run across a deal on pure antimony so good you can't pass it up. Or maybe you decide that you have a need for a more "precise" alloy. Or maybe you just get bored.:-)
Whatever the reason, know that it can be done at home. Alloying raw antimony with lead does not require high heat; it requires patience and basic knowledge of the process.
Antimony tends to combine with other elements much easier than with lead. This includes oxygen, so best results are obtained by having an oxygen barrier. It also means that pure antimony will alloy more readily with an alloy that already has tin or antimony in it.
The home caster will obtain best results using crushed charcoal or kitty litter (bentonite clay) to inhibit oxide formation. A more elaborate setup would use argon gas.
Doing it this way produces perfectly acceptable results, but takes time. A specialized flux can be used to considerably speed things up. But the LETS flux is very difficult to obtain in small quantities, and it is not only extremely toxic, but also very corrosive.
These are but a few of the reasons why Bill Ferguson no longer offers it. For that matter, antimony itself is very toxic, and it would not surprise me that offering both antimony and the specialized flux present unique liabilities.
This subject comes up occasionally. Here are some references:
http://castboolits.gunloads.com/showthread.php?t=65747
http://castboolits.gunloads.com/show...alloy+antimony
http://castboolits.gunloads.com/showthread.php?t=66147
All said, you're probably better served by using an alloy with antimony already in it. Various typemetals or the 30% superhard.
Good luck,
Rob