K-31's are slowly drying up but prices are still low. If you want to get Swiss, get a 1911 long rifle. Those have dried up but can still be found for fairly reasonable prices.
Don't worry about the 91/30 Mosins from the USSR. There were over 20 million of them made and they will NOT be running out. There were even hundreds of thousands of Soviet 91/30 snipers made. The TRULY RARE MOSIN-NAGANTS are the FINNS and certain unmodified M-91's from before the first world war. there's a comparative rarity chart at
www.7.62x54R.net but basically even the most prolific of the Finnish Mosins, the M-39, was made in vastly smaller lots than any Soviet Mosin. Any Finn you can find from the Winter War period, including Finnish M-91's, M-28/30's, M-27's and the like, are EXCEEDINGLY RARE AND INHERENTLY VALUABLE RIFLES. Only a tiny number were made, some lots numbering in the low five digits or less. They almost all saw combat. Thankfully for those of us who like these rifles, even most collectors are ignorant when it comes to Mosins and don't understand the enormous difference in rarity between Finns and USSR rifles. Later Continuation War Finns such as the M-39's, Tikka 91/30's and late model M91's are more common and many never served on the front line, but even these are orders of magnitude less common than the Soviet production rifles. The prices have not caught up with reality, but they will. You can still find the Rifle of White Death, the Civil Guard 28/30, for $300 to $400. It's true value should be closer to a thousand bucks, considering its legendary battle history and extreme rarity. But the market is full of bone heads who think a Mosin is a Mosin. DO NOT COUNT ON THIS TO CONTINUE! In the end the markets always correct themselves. I watched it happen with FN-49's, which went from being regarded as cheap Egyptian crap to rare collector's items as people figured out what they really were.
Military Mauser Rifles from South America, Africa and Asia are also going up and up in value. Grab any intact examples you can find if you see them for a low price. The near eastern and European lots are a mixed bag. The Persians are absolutely undervalued, but the Nazi marked K-98k's are typically OVERvalued.
Some off the beaten path US military rifles, esp. the Eddystones, have been very much undervalued compared with the Springfield '03's. But these prices are going up. If you can find an intact one for a good price grab it. Most of those left on the market have been messed with and bubbafied in horrific ways. Krag rifles also seem to be oddly undervalued.
But remember above all else, don't screw with the rifle. In the future most rifles that are un bubba'ed will be worth more than they are today. It wasn't too long ago that the M-1 Garands were as cheap as furniture legs.