All of the ‘best’ sources have disappeared.
I believe that an expensive Brewster window tube can be purchased from
Research Electro-Optics. It is a 5 mW class tube that is a replacement part
for their five-line HeNe.
Be prepared to be disappointed. Changes to product safety regulations make
it more and more difficult for manufacturers to supply bare laser tubes to
U.S. customers.
Please send a message to tank@usna.edu if you know of additional sources.
An ideal tube for a teaching laboratory would have a bore that is slightly
larger than the bore used in the commercial laser tubes. It is very doubtful
that such a large bore tube is available.