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 email@example.com 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.