There are definitely fewer out there than there were - I think alot of the really good stuff has just disappeared into collections. When I first got into collecting, 15 - 20 years ago, C-68 had just come into force and the market was flooded with unwanted guns, including lots of surplus. I remember buying Mosins (including Finn M91s) for less than $100 for dealers, and could even pick up an M91/30 at a guns how for under $50. Nothing lasts forever though, I was just thinking the other day about stuff that's been imported in the last 15 years you never seem to see anymore. I remember than Vz.52 pistols could be had all over for a good price and realised I don't think I've seen one at any price in 5 years at least. There were a bunch of Iraqi Tariq's imported around 2010 and they've dried up, and Soviet Nagant revolvers have quadrupled in price since they first appeared in 2008ish - I remember when you couldn't give them away! I remember $600 Turkish surplus Garands in as recently as 2007, Yugo M24/47s were around forever (you still see M48s though), same with M95 straight pulls. I think anything good quality sooner or later is going to get squirreled away and skyrocket in price.
In terms of real WW2, and especially WW1, guns, other than the Soviet refurbs, I think the large imports are long over and you're stuck buying on the secondary market. On the plus side, oddball ammo is so much more available than it used to be - the recent wide availability of PPU ammo is a godsend for the milsurp collector. Enfields are still out there, but they won't ever get any cheaper. I personally can't wrap my head around Garand prices, especially for the builds, but selfishly I bought mine for $400 in 2003 and it's not going anywhere! Finn M91s are still a great buy as a collector, and I think the M39s you see out there will jump in price once the recent import is all bought up (Jean Plamondon still has some nice ones) - M27s don't seem to command the same value and you only see M28s once in a blue moon anyways. I think the best buys still out there are the underappreciated rifles - Carcanos and M95 straight pulls can still be had for a decent price and ammo is actually available now, unlike when I started! As others have said it's definitely the best time to buy Soviet milsurps - M91/30s (including snipers), SKSs, SVT-40s and TT-33s can still be had from most dealers, but they won't be around forever. I remember when they weren't available (of course back then you still buy M44s for $80, try that now) and it'll swing around again.
As your collection gets more advanced it can be harder to find what you want - I'm at the point where I already have most of the easily obtainable guns I'm interested in. These days I buy a lot from auctions (there's actually been some really good milsurps at auction in the last year), but it's not a cheap way to get guns. Local auctions are more likely to result in decent sale prices than the big internet ones (Landsborough, Switzers) but always keep the buyer's premium in mind!
Believe it or not the time will come when it's hard to find unmessed with SKSs, since everybody and their brother (collector or shooter) buys one, lots of people don't clean them properly and at least half are sticking them in ugly Tapco stocks. There's still Swiss K and G11s around, but they'll be gone soon, and the refurbished VZ24s and Yugo M48s will probably dry up after that. The Chilean Mausers that came in a few years ago don't seem to have stirred up a ton of interest (probably the lack of interesting war use) but they won't last forever either.