There are several tour companies recommended for Russia but it may be hard to book a tour now
Denrus, Red October, Alla, SPB travel
You could check CC and see if you could join a group on your cruise.
click on the ship then the sailing date
Alexandr Pushkin, Carnival-Holiday, HAL-(old)Westerdam, Orient lines-Marco Polo, Royal Olympic-Olympic, NCL-Dream, Dawn-Princess, ACCL-Grande Caribe, Oceania-Insignia (3), Regatta(4) Marina (1) St Lawrence Cruise Lines-Canadian Empress
Scenic River cruise June 11,2012