Which alloy?

I've been loading and shooting the Montana Precision Swaging 535gr Postell bullet (.458 dia) in my Pedersoli 1874 Sharps and really like it. These bullets don't seem to be available now and may not be for some time, if I'm reading between the lines correctly? (Not available; no backorder)
I do have a new Lyman Postell mould, but not sure which lead alloy to use for casting this boolit. Midway sells casting lead as well as Rotometals. Any suggestions from either place?
FWIW, MPS says their Postell is 20:1 alloy, 9.25 BH

20-1 is a good alloy for the postel. Altho there's no flies in the ointment when using 16-1
Buffalo Arms ships it in 50 lb boxes.
John Walters will ship up to 60 lbs of it in a post office flat rate box.

Another plus for John. I have dealt with him ftf. Very helpful.

I use the 20-1 pretty much with all my BPCR rounds. It works very well and casts easily.I do cast the lyman 535 postell with great accuracy But I have converted my mould to nose pour also. I buy alloy ready to go from roto metals or MVA, both will ship in flat rate boxes.

https://missouribullet.com/results.php?category=12

Magic Alloy
6Sb2Sn92Pb Antimonial Lead
Modern equivalent of Lyman #2 Casting Alloy
Commonly also known as Hardball or Magnum alloy
Foundry Bullet Alloy with Certified Analysis 66 lbs. @$2.48/lb.
Price: $163.68

Smaller amount-16 lbs. @$2.55/lb.
Price: $40.80