As far as I know, this case cannot be formed from any other existing case. Legend has it that it was originally formed from .32 Wesson brass, but that case has been defunct longer than the .25-20SS. It can be made from 2R Lovell cases (a wildcat based on the .25-20SS) but those were only made from the parent case and also a very few runs of proprietary cases back in the 1930s and 1940s.
Bertram of Australia used to offer .25-20SS cases; quality was a crapshoot but they were all there was available. They may still offer them: try
www.buffaloarms.com or have a look at the Single Shot Rifle Assn. (google ASSRA) magazine; somebody may be making custom cases. Good luck; it's an excellent cartridge if you can feed it.