You can always buy 45-70 brass and trim it to the proper length. I once had a Whitneyville-Kennedy rifle. The only caliber marking was a plain 45. My 47-70 brass didn;t fit so I trimmed a few, loaded with the 300 gr; cast bullet using 10.0 gr. of Unique. The rifle shot just fine.
Paul B. IIRC, you don't have to trim a lot. Some of that Hornady 45-70 brass that has the bullets with the rubber nose are short enough to try as is. I wish I still had that old rifle. Weighed a ton but was fun to shoot.
Paul B.