7x57R cases are available from Midway and Grafs. I would think bumping it up to 8x57R would be a snap. I
think you can also resize .444 Marlin brass, but that would take some measurements to be sure.
Could you post a pic or two? Just curious about the rifle
.
Also check out
www.buffaloarms.com. They have brass and bullets for pretty near just about anything.