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

I have a Rogue Pharaoh. Great sounding unit. Has a very good Phono stage and headphone amp and an outstanding line level section. The stock tubes that come with the Pharaoh are JJ  12au7 tubes. I experimented with some 50 to 60 year old Mullard 12au7's, NOS. This, in my opinion, took an already good sounding amp to a much better one. The sound became more holographic. Air and space around instruments and voices became much more natural. The soundstage expanded in all directions. Pair the NOS tubes along with a premium power cord, doesn't have to be an expensive one, and you will be set. Depending on what you have in your system, you can experiment with Telefunken and Amperex tubes also. They can do similar things to the sound like the Mullards, The best thing to do is experiment.
Cheers,
Tim

Mr m, thank you for your input. It's good to hear you are also satisfied with the Pharaoh. Since I had no knowledge of tube equipment and reading up I decided that it was my best choice for entering this realm. I happen to very much agree with you. Out of the box I was impressed with the unit. Of course I have nothing to compare it to. I think I don't need to compare any longer. Someone else mentioned Mullard also. I'll give that a try when able. The Pharaoh is used with B&W 803D2, Musical Fidelity A3CD, vintage 1976 Sony PS4300 TT. I'm streaming audio thru an Oppo 103 using aux input. I'm also using the HT bypass where I can use my SS preamp for comparison. I will be removing this connection as there is no comparison between the tube pre and the SS lexicon pre. My SS system has been permanently assigned to HT duty only. 
Again thank you for your input and suggestions.

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.