As I understand it, cases made from .32-20 are not long enough to make the 'gas-seal' feature work. Case-length for the .32-20 is 1.315 inch whereas the Nagant case is 1.53 inch long.

I have a nice example of the NCO's Nagant Revolver (single-action only- the officer's model was double-action), and dies for both deep-seating in the correct-length case, and for the converted .32-20 case. So far, projectiles have proven to be the problem (they are light, and from memory they are not .308), but I really haven't had the time to devote to getting mine up and running.

I picked up a box of Bertram unprimed brass a few years ago. If you can't get Fiocchi or the Serbian stuff Huvius lists above, you could spin-out the .32-20 to correct length on a lathe. Slightly thinner case-necks would probably help with deep-seating anyway.

My thinking on this cartridge has always been that if you can't achieve the gas-seal that made this revolver what it is, might as well not bother. The ~100gr bullet only gets 750-odd fps in short cases, whereas 1100 fps is achievable with the gas seal by all accounts.