There's nothing unusual about the picture tubes for these monitors (Atari/6100), standard off shelf tube that's been used in all sorts of applications over the years, vector and raster monitors and TV's.
The yoke is unique, and you would have to swap that over to a replacement tube.
"New" (although sometimes they really are "reman") tubes have been available right along. But I am starting to hear rumors that the usual tube vendors have exhausted stock and are listing them as "backordered" with no scheduled in stock dates. The common part numbers for these tubes are still listed as being available, but I know somebody that called Richardson for one and was told not in stock. If you want to go the new tube route, you might have to do a little chasing.
D