Here's a link for all of the BMW dealers in Russia:
Мотоциклы BMW в России
The closest one to you is in Krasnoyarsk. The bad news is that these dealers carry virtually no parts, and to order something from Germany takes 30 days. But give them a call, who knows, maybe they'll have a sprocket.
Another HUBBer (KLRKestrel) is currently stuck somewhere out in Siberia, having ordered a F650 part from the US, but apparently it is currently stuck in customs in Moscow. I seriously doubt it would be quicker to ship to Magadan, which is literally "at the end of the line".
Another option is to find a Russian mechanic to fab/modify a similar sprocket. Russians are, by necessity, great at stuff like this. There is a moto club in Vladi that could probably hook you up, you'll probably find them if you search this site.
Bottom line is to bring BMW parts which commonly fail, or need to be replaced, with you to Russia...