I recently purchased an original Frank Wesson No 1 Creedmoor rifle. I was told it is chambered in .45 Turkish Peabody-Martini. Does anyone know where I can find some brass or brass that can be formed into this cartridge? The rifle is in very fine shape with a mirror bore. It has the original front sight and both rear sights numbered to the rifle - one is for the tang and the other for the butt.  I wouldn't have bought a rifle with such a weird cartridge but I couldn't resist. Sure would like to shoot it!

Lookk for the big brit NE cases - 470 NE, 500/465 NE,...  Most with a .50 base will be ok.  Actually, 50-90 will also work for forming and might be the easiest option.   1st operation is to trim to OAL, deburr (important!  helps prevent case collapse), then anneal the brass far enough down to where the shoulder will be.
Next lube just right (enough, not too much) and size in 2-3 strokes in the sizing die.  retrim, deburr and you're ready to go.

Strange?  Yes and no - after all, Providence tool madelots of them for the Turks, and the surplus was sold on the domestic market.  It also is a very good cartridge indeed, exactly right for target work.  Kinda like a no. 2 Musket with the shoulder a bit further back.

Thanks all for the replies. I will chamber cast and try the 50-90 option since I shoot a 50-90 Shiloh.