The Citystar-ticket is only sold at railway stations in Slovakia. You might organize your trip so, that you travel via Bratislava.
Buying the Citystar-ticket in Bratislava is no problem, but requires exact knowledge of the border points and the exact route to issue the ticket. Some knowledge of Slovakian language would also help sometimes.
You can also get the Citystar-ticket from a German train-ticketing agency located in Berlin:
You can e-mail them to firstname.lastname@example.org
They will be able to get the ticket (including necessary sleeper reservations) and send them to you.
Also they can assist you with tickets for international trains from Russia into China and Mongolia.
For questions regarding the Citystar ticket to Russia you can also directly contact Slovak railways.
See also http://www.zssk.sk/en/contacts