And any international ticket counter will easily sell it to you.
You mean any international ticket counter within Russia or in all CIS countries & Baltic states?
I checked also RZD site and found the connection there, I was even able to select the wagon and number of travellers but as I am not registered the system kicked me out. Nevertheless, it seems that it is possible to buy tickets only for domestic travels and international travels to CIS countries
The system shows availability for given train and date but not the price, is it possible to get the price quote somewehere?