1.) 8 pin tube. Standard 6SN7 (all variants GT, GTB, CV181, etc.), 6H8C or 5692 except for 12SN7.
2.) If you want NOS only, go to tctubes (period) com or if you want new production tubes (EH, Tung-sol reissue, JJ) and NOS in one store, go to tubedepot (period) com. If you want used tubes, go to ebay of course.
3.)The 5692 and the vintage GT (not the newer GTA,B) gives one of the widest soundstage and rich sound. If you want overall smoothness, go for the CV181 from Psvanetube
4.) Select match pair from options.
5.) Newer tubes are better value than NOS, but the sound quality depends on the amp. With Saga, NOS sounds better than new tubes, but they range from subtle to noticeable right away.
6.) see reference 6SN7 headfi
http://www.head-fi.org/t/117677/the-reference-6sn7-thread