Indeed, normal HeNe lasers are one piece with the mirrors also acting as a barrier to the outside world. Changing a tube like that would almsot require the resources of making one from scratch (thinks like pure gasses, vacuum system, bake-out ed).
I have two Spectra Physics SP-120, one barely living and one reasonably fine. They are very old, but external mirror HeNe's none the less.
The one at the university is probably 3 years old, it's used in a practical course I helped setting up. It's made by Lasos, although I can't find this tubes with brewster angle windows at their website.