Have a friend who is interested in that part of the world, he has used the tour group Voyages Jules Verne. www.vjv.com. They are a UK based company, the headline cost of the tours includes airfares from the UK but they also quote a price excluding airfare so you make your own way there. My friend's next trip with them is to Kazakhstan and Kyrgyzstan - I'm interested to see how that goes …..
There are several more UK based tour groups who go to that part of the world, I can try to list them if that is of help. But I'm sure there must be many more options in the US.