I think Warlord may have some M1936 helmets in their Soviet line, sometimes it's hard to tell what things are meant to be with Warlord.

Not sure if there are enough to kit out all figures though.
Their plastic Japanese are sculpted in tunics so they could be used. Leaving aside the casually racist facial features of some of the sculpts, they aren't the worst offerings out there. Tanks for the period abound, no shortage of choice there including some lovely stuff from Rubicon and Trenchworx.
Always mystified me why nobody does decently sculpted, dedicated Soviet ranges for Nomonhan and The Winter War. Surely there must be a market?