That's a good question Bertieng!
I do know that is one of the rarest parts around, you see the crowns for sale from time to time (there was a complete used 8mm crown & tube sold recently on ebay for well over $1000), but rarely see tubes for sale. Doubtful that there is an aftermarket version for it too. Curiously do you have a gen 8mm Brevet crown? If you do, your best bet may be to have one made.
I have a DSN brevet crown for a Bond Sub project, but it will take some work since the tube is huge and wasn't intended for a sub case. This project is on the backburner for now.