Old Fuff,
Increasingly Numrich, aka GPC, is selling the small quantity items via Gunbroker and Ebay. Auctions on rare parts probably make them more money than waiting for someone to call them. They are now on the biggest sellers on Gunbroker and Ebay (for gun related stuff). I suspect at least a couple of the other big dealers for gun parts/ odd variants on the auction sites get their stuff direct from Numrich or Sarco.
Regarding the O/P's Nagant issue, R guns used to sell quite a few of these and might have one or two parts guns or simply parts ratholed or just keep checking Gunbroker for "parts guns". I have seen Nagants among them from time to time. The parts guns often turn up being cheaper to purchase than the individual items such as a revolver cylinder. There are a couple of sellers on Gunbroker or Ebay that buy part sets from the court order destruction of firearms and sell them as a set for a particular firearm (minus the frame/receiver/ATF serial numbered part which was destroyed) as well as those selling individual parts.
Fleabay aka ebay is also a website that can be useful but you have to wade through a lot of dross to find a bit of gold or be prepared to pay out of the wazoo with buyitnow option. The cheapest prices generally come from general pickers who sell a bit of everything but know nothing about the items that they are selling. Likewise, their part id for the Nagant cylinder may be something like vintage gun part thingie for a revolver. Gunbroker is the next best auction source but usually has experienced sellers that know what they are selling and it is priced accordingly. Other auction sites are hit or miss.
Last, but not least, for milsurps at least, the trading boards may be useful on website forums such as gunboards, milsurps.com as mentioned above, and other places. A lot of times members on these boards either have a spare part if you put a public beg for a part or disclose where you might buy one.
OP you might also try a few of the parts wholesalers who are not online (tends to discourage massive buying the parts then reselling them for more on fleabay etc. ) e.g. Jack First and Lee Gun Parts in Texas. I know for a fact that this has caused places like Numrich and Sarco to be depleted on common parts for popular models as individuals buy the stuff then try to resell it for more on auction sites. Springfield Sporters and some others now are imposing limits on purchase amounts for this reason.
I found out more than I needed to know about this stuff because I developed the odd hobby of restoring butchered and maltreated milsurp rifles after being house bound for several years. Couldn't shoot so spent my time piddling with multiple rebuilding projects over the years. I have used all of these methods above to get parts as well as haunting old-time gunsmith's shops who often have all kinds of stuff in parts boxes.
Good luck, O/P.
