Besides military issue holsters before and after WW2 many smaller firms offered generic military type holsters for both revolvers and autoloaders. This sort of holster would be bought up by just about any user to suppliment the standard military issue.
I've seen these offered for sale in the past but they were made in such limited numbers I doubt many could be found now days.
I would think that from the general dimensions of the Nagant that a holster made for the S&W M&P should work, and possibly the Naval aviator style shoulder holster meant for the Victory Model. Replicas of the shoulder holster are available.