Eligibility to Marry document???

I am a U.S. citizen and plan to marry my fiance in Saint-Petersburg, Russia. He is a Russian citizen.
Where and how can I obtain a document/certification/anything that proves that I am single and have never been married/divorced??? It is required by Russia in order to be eligible to marry.
I see that the USA doesn't have this type of document, so what should I do? Is there an alternative option to getting this doc? What have other people done in a similar situation?
Thanks so much!