Holmes,

I have a 375 Flanged Magnum in a Rigby Sidelock (the proper English one, not the US copies)

You will love the calibre - don't let people persuade you to go 9.3 x 74.

Brass and Dies and Bullets are readily available for the 375 Flanged Magnum
and will be for as long as I can see. Brass is available from at least 3 Manufacturers.
If you ever have a problem getting some, email me as I have 200 + and can spare a few
or can put you in touch with the manufacturers.

Can't recommend a Modern maker - will leave that to others who know the Searcy's etc.
I would however stay away from the Krieghoff due to the Cocking / Decking system.

Good luck in your venture.

500 Nitro