brass is easy
https://www.starlinebrass.com/445-super-mag-brass
and in a pinch 30/40 craig brass can be cut and expanded and used
even some brands of 303 brit brass can be cut and resized to make brass
yeah, its not the easy choice, at times but in my opinion well worth the time and effort,
simply because you have something both unique and noticeably more powerful than a 44 mag,
that can still shoot 44 mag cartridges for practice,
but has a very noticeable increase in range and power if properly loaded