That one is a difficult one to acquire. I talked to a designer of mike pre's and he expressed concern over using a valve whose supply of acceptable examples is small and getting smaller. I was warned to watch them for noise. I would caution buying them w/o a noise guarantee.Radio Electric Supply has them for $65 each and a guarantee would could be confirmed ahead of time. I have gotten excellent service from them, and for a rare and difficult tube, I'd go nowhere else.
cheers,
Douglas