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

It's true that as availability of NOS tubes goes down, prices do go up. And the supply of true "New In Box" old stock is diminishing, but there is still a great supply of NOS tubes that are removed from electronic gear from military, medical, industrial sources. Although they are not really new, they have either minimal or zero hours of use, and will test as new.
That's why anybody who's into high-end tube gear needs to own a tube tester to check their purchases and their personal stash.

Your question is too general to give a specific answer other than to say that price is a function of supply and demand.  Just look at vintage Western Electric 300B, United 845 as examples.

In the link on Brent's site, click a tube that interests you and you will see his detailed descriptions on the history, country of origin and sound characteristics of all the different tubes used in the industry today. It is very useful info and his offerings are top notch.
http://www.audiotubes.com/

Not all NOS tubes are expensive. And IME, with the few tube pieces I have, I’ve found that high priced NOS tubes don’t necessarily equate to best performance. As usual with audio gear: it depends. For example, I had an expensive pair of NOS tubes in my preamp which sounded great. I replaced my power amp and ended up having to replace my expensive NOS preamp tubes with inexpensive NOS tubes because they sounded far better (to me) with my new power amp. You simply cannot predict what will work best in your setup until you try it, so you cannot predict for certain that your future NOS tubes need to be expensive. Just my $.02.

I agree with the above comments saying it depends on the specific tube and the application.  There are some very rare and high priced tubes, but overall there is no problem finding great sounding tubes.  It's been my experience that price doesn't fully equate to performance with NOS.  A Bugle Boy or Mullard may be at the apex, but a Brimar, Mazda or even RCA at a quarter of the price can be excellent tubes.

IMO, unless you are buying on Ebay or scavenging electronics at estate sales, you don't need a tube tester.  Getting an accurately functioning testers and knowing how to properly use it is a non-trivial task.  A good vendor can do all that work for you.  I'll plug Brent Jesse, but he's not alone.