I have had great success with Brent Jesse
http://www.audiotubes.com/. His website offers invaluable information and Brent really knows his stuff. Pricey but hi quality with guarantee.
I have purchased the bulk of my fairly large NOS collection from a great local source to me in AZ. He calls himself Arizona Tube Supply and has an Ebay store. His direct email address is Dave Hollander:
n7rk@cox.net
I also recommend a seller in LA by the name of Steve Song. He advertises quite a bit on US Audiomart and seems to have an unending supply of wonderful NOS Mullards...mC1's, F-91 Longplates (my favorites) and Telefunken smooth plates.
stevesong111@gmail.com
Have fun!