Well, first of all I´d like to point out that I shouldn´t do it. No, I really shouldn´t tell you where I buy my tubes. Because nobody with an affinity to tubes tells his sources. All tube hifi enthusiastics need NOS tubes and the number of these tubes does not grow, I assume... if you have a real good and reliable tube dealer you have to guard him against others as if it were the Holy Grail. Normally.
Yeah, we tube hifi nerds are a very special species, I guess... Anyway, I recently bought a pair of NOS 2A3 tubes (post following soon accordingly) at Langrex/UK. After some days and several hours listening time one of the tubes made some trouble: it was strongly blue illuminated (not a problem in principle), got lower from time to time and was swooshing too intensively.
Of course first time I was a little bit disappointed the more so as the tubes had a very fair market value but regardless of that they were expensive. I contacted Langrex and - hey, I really was pleasantly surprised: perfect communication, quick reply, no discussion or doubts but an excuse and the announcement that Langrex will send a new tube. I should dispose the defect tube, they told me...
In the end I can tell you that such a good service is not the general standard; unfortunately I made some contrary experiences in the past with some other dealers. That´s the reason why Langrex is highly recommended, if you ask me. Besides, they really have a huge stock of NOS tubes. As far as I know no more 2A3 tubes, well, that makes it easier for me to recommend Langrex to you openly... ;-)