Hi, when it comes to buying NOS or used tubes, I rely on Brent Jesse Recording and Supply. He grarantees the products he sells. He also has a wide variety of NOS tubes to choose from. Another thing I like about him is he takes time to share his wealth of knowledge which to me is priceless. His website is very innovative. Here is a link
http://users.rcn.com/brentjes/bjtubes.htm
http://users.rcn.com/brentjes/bjtubes.htm