I'm not sure if this info would help you, but such things can be easily found on different flea markets and bazaars in Russia, Moscow or St Petersburg. I bought a couple of rare filters in Petersburg, as far as I remember, at the Udelnya flea market. https://petersburg.expert/blog/udelnaya-flea-market/ Here you can read more about it, opening hours, tips for bargaining (it says here that sellers usually don't know the true price of their goods, so maybe you'll find something cool)_etc. Or maybe some of your friends are going to Russia? Maybe it's possible to ask them?
That flea market looks like an interesting place to visit, with enough stalls to make you take a couple of hours to look around the place. Like all flea markets of course, there'll be a lot of barely useable stuff, but if you're lucky you'll be able to some good stuff, and maybe exactly what you're looking for on a particular day. It's quite possible to find something you didn't know you wanted which you just have to buy.
I know this won't help you Phil, as you posted almost 5 years ago, but people may want to be careful that the filters they buy actually have a filter thread on the front if they use a threaded lenshood. I've had an Orange Russian filter that didn't have a thread and I hadn't checked as I'd always owned hoya made filters that do, so I had to replace it.