There are no Russian aircraft that I know of that are currently certified for Airline operations in the United States, not even the modern Tu-204. Yes, they are allowed to LAND here if certified in their own countries, and they are sometimes used in the US for cargo charters, with foreign registration and certification.
And some individuals have registered MIG-15 and other Russian planes as experimental aircraft for personal use.
But none have been certified to be owned and operated by US carriers for passenger service . That kind of certification is very difficult and expensive to achieve. So forget the Yak-40.
There are, however, a bunch of cheap and very efficient used turboprops out on the market. They have been displaced by RJ's. I have thought that perhaps you could pick up a couple of these and start service between city pairs in the Northern Great Plains that have business connections. Fargo, Bismark, Boise, Cheyenne, etc. The operating cost of a 72-seet turboprop might be competitive even with driving if you did a good job in it (I have no expertise to back this statement up - it is a pure shot in the dark
). High frequencies, cheap prices, ya know - the Southwest model - regional style.