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"?

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. 

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.

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.