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

Very new to this game, just purchased my first tube pre amp. Been ss forever. My question is, what is the difference in plates on a tube? My pre calls for a 12au7 long plate. Are there any other "plates"?

The plates are the black or gray (usually gray) rectangular or square objects that take up most of the space in the tube. Long plates are usually found in older tubes, early (1950s) versions of tubes like the 12AU7. They may sound different from the later short plate or box plate or square plate (three different names for the same type of plate) versions of the tube but any 12AU7 socket that can use a long plate should also be able to use a short plate.

Tell us what brand of preamp you have and we may be able to help you get the right tubes in it.

Just purchased a Rogue Pharaoh integrated and am not familiar with tube jargon but read that "rolling" tube can be fun experiment if you know what you're doing. 

Welcome to the world of tubes.  The Rogue should be a nice sounding integrated.  I know we have some Rogue fans here.

Tube rolling is just trying different tube brands in your amp to tune the sound to your liking.  There are also variations on the 12AU7 like the 5814 and many others that work where a 12AU7 is called for.   So there are a lot of tubes that you can roll into your amp, but it's not a cheap form of entertainment.  The Rogue has 2 12AU7s from what I can tell and any 12AU7 should be able to work in your amp.  What brand of tubes did it come with? 

Why don't you start a new thread asking Rogue owners what tube brands they found to work well  in their amps.  They can probably give you a better educated opinion than I can.

Thank you all for your replies to my inquiries.  I love the forums as it provides a very diverse environment and opinions. I am learning a lot on my new tube endeavor. Thanks for enlightening me on my "plate" question. Thanks for your suggestions.  
For a 40 year SS veteran this is a big change. All my SS experiences have evolved from the early days of getting the best, in my opinion, to top of the line in the later days. I am satisfied with what I have. Now, after hearing a tube sound, I can honestly say I had no idea of what I was missing and regret the amounts of money spent on improving my SS environment.  I find I no longer listen to my SS system and favor the tube. Just last night I realized I had been immersed into the sound for 6 hours and time stood still. Did not realize it was 11PM before I retired to bed. It's a good thing I'm retired and have that time on my hands. I am truly impressed with that "tube" sound I always heard of. What a great way to enjoy my golden years.