Floortrader and Redwoodrandy both referenced the key places to get those stamps and postal history, Loral (russianstamps.com), Raritan and Old Louis. I highly recommend them all.
The only note I would add is in regard to Old Louis. They shipped from Ukraine even though the auction is US based, and it could take a while to get the goods. That may have changed now though. They were talking about shipping from the US.
I would also add Cherrystone Auctions to the list. The owner Paul is a world-class expert on Russian material and his certificates are legit.
They all stand behind their material.
I DO NOT recommend the larger
ebay dealers of Russia. One offers certificates that frankly are worthless. I strongly suspect shill bidding and other games with at least one seller and a lot of MNH items I purchased were found to be regummed when I sold my collection through Raritan. Stick to the non-Ebay venues and you should be very satisfied.