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

gillatgh, Unless you plan to become a tube fanatic, when you do start to roll tubes buy them from a reputable dealer rather than off ebay.  There are good tubes available there and they are probably cheaper but you need to know what you're looking for and have a tube tester to make sure they are as advertised.

Good tube dealers will talk to you about what sort of sound you would like to get with your new tubes and offer educated opinions on what  might work for you.  Some good dealers are:

Brent Jessee - he has a lot of information about tubes on his web site if you would like to study up a little.

Andy Bouwman at Vintage Tube Services - He can be a little slow getting your tubes to you but many people feel he's the best.

Kevin Deal at Upscale Audio - very knowledgeable and very straightforward with his advice.  He knows what he likes.  Some people don't like that but I enjoy talking with him.

There are others that other people will recommend.  That's just 3 to get you started.

I forgot to mention about a good tube dealer in my post. I too, get my tubes from Andy Bouman of VTS. This gentleman is extremely knowledgeable about tubes. Especially NOS. Yes. Different makes of tubes have specific types of sound and you should match them to your system accordingly. 

Jim McShane is the fourth name that should go in that shortlist.

Kevin Deal at Upscale Audio

+1  I have known Kevin for over 30 years.  The dude is a tube guru.  He has never steered me wrong.

If you are interested in reasonably priced cryogenically treated tubes, contact Ron Sheldon at Cryoset.  His cryo'd new production Gold Lion tubes sound great and seem to last forever.  http://www.cryoset.com/catalog/index.php