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

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.

Analogluvr, I agree, or I wouldn't even be thinking about tube equipment, especially power amps. But I also value dynamics and clarity, so all this requires very good tube electronics. 
Extra cost is fine, just within reason cost, good things are worth paying for. By doing that you also support those who make those good things. $10k used minimum for the tube equipment if your speakers are efficient enough and room not big. Until I can afford this I will stay with decent ss integrated.

jond, I don't know. I guess, I would gravitate towards Atma-Sphere, Lamm and possibly Allnic and VAC equipment. Ypsilon would always be out of reach, I think, except maybe used integrated. And I mean Lamm hybrid power amps.