Keep an eye on the Internet gun sales sites like Gunbroker.
Go to
www.russian-mosin-nagant-forum.com & do your research, might ask around there.
Understand there are no new PUs.
Any PU you find for sale will likely be a post-war refurb, most with post-war Russian forced matches between scope & rifle. Those may be legitimate sniper Mosins, and the scope may be a genuine PU from the period, but the two may not have gone through the war together.
Relatively few true Mosin snipers survived the war with original PU scope mounted.
There are many fakes, running from rifles that genuinely were "snipers" at one point now wearing repro glass, to cobbled up never-were sniper rifles assembled from a standard Mosin with mismatched bolt numbers, a repro scope mount & repro optics, and in all sorts of conditions.
If you buy from an individual, you may not be getting "the real thing".
What to look for?
A stock with a mount cut-out that doesn't look like it was done last month.
The correct mount.
The correct scope.
Tight barrel bands. (Probably impossible if buying sight-unseen.)
No severe barrel rust below the wood line. (Same problem.)
Good bore. (Same problem.)
Don't be thrown by crudely-applied electro pencil serial numbers on the mount that "marry" the optic to the rifle. Those are legit Russian refurb markings.
Don't be thrown by a "new" serial number on the receiver that doesn't match the original serial on the barrel, bolt, triggerguard assembly, and buttstock plate.
Importers had to assign new serials on the receiver for importation, since the original serials were not on the receiver as required by US law. You'll also find the importer's company name on the rifle somewhere.
Do expect the original serials on barrel, bolt, triggerguard assembly, and buttplate to match, in original stamped number fonts.
Understand that the odds of you finding a completely original unaltered PU Mosin that was not refurbed after the war are very slim.
The Russians rebuilt & replaced as necessary before putting the Mosins in storage & did not care about keeping things "original", beyond keeping serials matching when possible.
They were refurbing for possible re-issue, not for collectors.
Denis