There were several awful cases of Russian children who was adopted by American citizens ending up dead (in many cases it was due to neglegence and once the child was beaten to death). One 'mother' simply put the adopted child on the plane and sent him (or her - I don't remember) back to Russia with a note.
So there are very strict rules of adoption right now. In fact, an adopted child would keep its Russian citizenship till he (or she) is 18 years old (by the agreement signed just recently by our presidents). There will be many checks and you'll need a lot of proofs that you won't mistreat the child and generally - prepare to go through a bureaucratic hell. The adoption agencies are looking with suspicion at every new potential parent. Especially when they happen to be Americans.
Still, I don't know if it is such a big problem to find the agency because just one search on Google revealed this.

Nevertheless, I advise you to come here, to Russia and go looking not through the agency but through numerous orphans' houses here, in Russia. If you CAN travel here I think that in the end it might save you the money you would pay to an agency.