How expensive are best NOS tubes ? And how difficult to find ?

They aren't going to get cheaper, but I'm not sure you should base your buying decision on the cost of the tubes. And agree, it is harder to find some tubes now than it was even 5 years ago. Most of those small tubes last a long time, but if they are pulls, rather than truly NIB, who knows?
I only have a few tubes in my main system that are costly- the Tele 12AX7 in my Lamm ML2- well worth the price every few years- as you said, most of the other tubes Vlad supplies, and they aren't terribly expensive. The real DR Reflektor 6 H 30 is costly- I use that in my line stage. And I roll the rectifier in the Allnic, but I regard that as a matter of fun- you can get a decent old rectifier for little money, or a lot of money (I'm using a GEC U-52 right now).
I shelled out quite a few bucks for tubes in my vintage system, but a matched quad of GEC KT 66s! Genelex EF86s? Vintage Mullard rectifiers? All worth it. 
I used to replace the Pirelli P-Zeros on my sports cars every 5 thousand miles. That wasn't cheap, but it came with the territory. (I don't even know if that tire is still highly regarded, but it was pretty sticky and wore pretty fast). 

Perhaps I would not "base" my buying decision on that but it sure would be a significant contributing factor. Even more than the cost, unless it is truly "stupid", though I would not enjoy getting used tubes instead of new and having to run around the globe just to get some of those Mullards and Telefunkens after couple of months of the hunting expedition. Tube rolling may be fun but chasing them is not.

Inna  I can't see your cost being that high. But who knows if I sat down and tallied mine up I may crap my pants :-) 
But solid-state is not an option for me, the things I value such as dense harmonics, presence, sounds staging is all what tube gear excels at.  Basically I find tube gear to excel at natural musicality. And then there's the cool factor!

 And I have had good luck with Chinese power tubes as well 

@inna  You really haven't mentioned a specific tube type you are looking for but the type really does influence cost. Tons of good not terribly expensive tubes in the 6DJ8, 12ax7, 6SN7, and various other categories mostly preamp/driver tubes. You can also pay a lot for these types if you want. NOS power tubes tend to be more $$ especially DHT types. So the answer as with most things, it depends.