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

Not every stage in a preamp or amp needs NOS tubes. There are phase splitters and cathode-followers which have less sonic affect on the circuit. New production tubes are cheap and also sound good as power tubes. There are some really good quality Russian tubes now, and I believe they will continue to get better sonically.

Some of the high quality Chinese tubes cost as much as some NOS. If given the choice, I would buy NOS from a trusted dealer.

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.

That’s exactly right, randy. Finding the cream of the crop will cost the consumer.
I have a very revealing single-gain stage preamp and needed to return some microphonic 6SN7 tubes to Vintage Tube Services. Andy replaced them and told me that he needed to test 60 tubes to find a pair that he considered high quality, (and they fetched a premium price).