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

 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.

some years back a fella I knew then had a pair of Amperex pinched waist 6922s he said he could sell for $600 each. I'm sure more expensive ones are around too.

Rolling fun. Chasing, not fun. Absolutely.

Although disposable items, tubes are investments too.

A true luxury is being able to retube the appliance every 8 – 12 months. It is usually NOT necessary though. With normal use, only on when a session is pending and off afterwards, several days per week, in that third year I might want to replace them.

Having that second set or a tester around will tell you exactly what to do.

Finding someone in your area with tube gear maybe both can buy into a tester and share it when neded. One can always hedge their tube bets.

If you have your Goldilocks gear, Buy more than one set ASAP. Don’t wait for them to burn out and need replacing. Buying two sets of those tubes which actually influence the audio, not each and every tube in the amp or ??? with 2017 money is better than noe set in this year and another set in say 3 or 4 years.

Soon enough Its gonna come down to buying gear that sounds great as is. No rolling required. Then of course, buying at least a couple sets of the ones that matter.

One thing which will help if possible is keeping the ambient heat lower when operating the gear. Heat is what is killing them. Not passing signals.

Various vendors have various QA standards. Some higher, some not so high. Some have several testers, some only one. Some don’t have associated gear to check them in at all.

Checking out a new vendor IMHO should be done with less pricey tubes. Just to see what the seller feels is a quality, low noise, etc., item. If possible.

The point made previously on gathering many and using a few is a really good point. Especially military com gear.

When I was in the Navy, at times we could not even order new tubes. We had to order the entire unit instead!! Whoa. Or go out into the world and buy them open purchase if we could find them.

Lastly, if the Goldilocks amp needs to be sold later, trust me selling it with NOS spares is a big plus!!

Or keep them for some other amp later on. They ain’t going down in price and if not in use, well, isn’t that what NOS means?

I’ve several barely used RCA NOS from the 50s & or 60s, Amperexs too. Even a NOS pr of Amperex PQ US 7308 whites, I’ve not used in years, and ran them only 30 or 40 hours before I sold the preamp I bought them for originally.

Buy more than what you need at the time when possible. Especially those often used by designers, like the 6DJ8 CLASS, 12AU7s, AXs, ATs, etc. any true Mullard NOS, Breyner, telefunken from W Ger., Tungsol 6L6, 5881s, etc.

beware the Telefunken dupes and Mullard Russian reissues.
Good luck.