I have a Rythm integrated which uses 6C33C tubes. They are long lived tubes, but can fail. I have purchased spares on fleabay from fet.audio, a seller located in New Jersey. I believe he is the fellow @armstrod mentioned in his post. My dealings with him has been smooth and I am pleased with the quality of tubes.
That said, I purchased a set of Tungsram tubes from the Ukraine in April and received them in about 10 days with no problems.