I cast my own, powder coat, lube,Lee Alox,45-45-10, etc. and run a BHN of 12- 15 for 45 ACP, 10mm, 9mm (RN & TC for auto), 44mag, 38 SPC and .357 Mag (WFN, SWC & HP for revolvers) from mild to wild (full power) loads. Lyman and Lee have manuals that covers load data, hardness, etc.
Anyway, bullet fit is king over hardness which means you need to slug the barrel and be .001" over bore diameter. The only hard cast I cast is for my 450 BM (BHN of 21) and that is because I hunt in brush.
Matter fact, my 30-30 cast alloy is WW and pure lead mix w/ 3% tin (170gr FN sized to 311", gas checked and PC'd) with a BHN of 10 using LE powder which I run them at 2100 FPS with no leading issues. The bore slugged at .309" and was getting 3-5" groups with jacketed bullets. My cast shoot 1.25-1.50" groups @ 100 yds.
It's been at least 6 plus years since I bought any but google cast bullets and tons will come up.
FYI- I had to send my Ruger GP100 in to have the cylinder opened up as the bore was .3565", cast sized .358" and the front (some) of the chamber openings were .356" (checked with a pin gauge) which squeezed my bullets down and caused accuracy and leading issues.
Montanna, Missouri, Midway, Brownell's, etc. sell decent cast bullets.
ACME Bullet CO. was my source before I started casting and they would custom size them for me. Also, cast boolits forums is a good place to buy casts from some of the vendors at a good price with custom sizing, coatings, hardness, etc. too.
Powder coating them yourself is easy and there is lots of YOUTUBE, Rumble, and Reloaders Network videos about PCing, sizing, etc.