Try snscasting.com they offer several styles and calibers and you can try sample packs.

Henry did come out with a 41 mag Big Boy Steel. I also would like to see one in 10mm.

snscasting .com has coated and uncoated 220gr. in quantity and sample packs

Lancer mags are the way to go as they have no separation rib down the front which limits length on single stack 458. Metal GI mags also work but may need slight modification for some rounds.

I have found that a case gauge is almost a must to ensure your rounds will go into battery. Once I ordered and rec'd mine my range trips were a lot less frustrating.