It appears that Upscale Audio has suspended tube sales from their website. I suppose you have to call and speak to a sales person. But my sense of things is that unless you have a really good story you may only be able to get only a replacement tube.
TubeDepot’s website shows many tube types unavailable. Some of the more expensive tubes are available but only in very small quantities.
valvesnmore as of last night, seemed to have more tubes. But certain types might be available in small quantities or unavailable.
Good luck.