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). 

@inna- I do my shopping from my home- known vendors that are reliable, and some I discover along the way. Tubemonger, who I hadn’t dealt with before had never used Genelex EF86 driver tubes (used in the old Quad mono amps) that were flying lead and he and his partner converted them to conventional tube pin socket type. So, I was buying a tube that i knew had never seen any play. And they were cheap. I usually use Brendon at Tube World, but some of my friends have stashes of tubes they’ve bought over the years for equipment that they might no longer even own. (the downside of being too forward looking).
It’s like buying a obscure pressing- the chances of finding it in your local brick and mortar store (and there are very few b&m tube stores) are slim- once you’ve done business with a source for a while-- and there are several, the question is simply whether they have the tube(s) you want. Priority shipment in the US- voila - a couple days max unless you are in a hurry.