Hi Mr. Tennis. I would do a google search. E-Bay would probably be a good place to look. These tubes are not readily available in any case and I expect you will pay a premium.
Rfogel8, I too have purchased from e-bay dealers with good success, their feedback rating is important when dealing with them. It is always wise to purchase from dealers that show published test results and better yet, offer a guarantee and avoid like the plague tubes being sold "as is" at premium prices. My comments are based on experiences I personally have had with so called repudable dealers. Tubes I didn't test based on recommendations and that were spent, beyond their usable life yet sold as "NOS" because they were a premium, desired tube. One dealer I had two such experiences with. One time I'll give the benefit of the doubt two times it is no longer a coincidence. My point is really directed towards new guys, be careful and deal with reputable dealers with test results. If you really get into purchasing a lot of tubes invest in a tester. For what you can spend for tubes it is a modest investment. I use Andy as the best example of someone who's reputation is unimpeachable and will educate and be fair in pricing. Another good reason to own a tester is to test periodically to make sure your system is operating optimally. Some circuits use tubes faster as they are biased at higher currents, particularily power amps.
