I bought a bevy of Electroharmonix tubes from www.tubedepot.com. They had the best prices I could find for new production tubes, and their shipping was super fast. Can't say about NOS tubes, though, if that's what you're looking for.
I would highly recommend Andy Bouwman at Vintage Tube Services. You can check out his website at www.vintagetubeservices.com. He is located in Rockford, Michigan.
Andy has as knowledgeable about tubes as anyone you could find. He basically has been a student of the vacuum tube since he was a teenager.
I have purchased a number of NOS tubes from him. The results of adding these to my system has been nothing short of miraculous.
Every tube I've purchased off eBay has been bad. And one pair of very expensive tubes that I purchased here went bad inside of two weeks. FWIW, I only go with the bigger suppliers now. Among them:
If you don't have a tube tester, and complete confidence in the seller, be careful. NOS tubes are a delicate lot.
All the best,