I actually don't know if this is correct, but I was under the impression you could not book trans-siberian railroad tickets from the US. I know on the Europe side, it was recommended to have a travel agent book you a courier service to arrange the tickets in Helsinki, Finland, but am unsure about the China/Mongolia side.
While I am pro independent travel, I think this might be one of those things you should speak with a Eurasia travel specialist about. Good luck! almost 5 years ago