Braun have cancelled their photographic goods long ago. I'm quite sure you would not get any spares from them, let alone such old part.
You better either look for such old flashlight to take off the tube, or consider spares from current studio strobes manufacturers or have a tube custom made.
I'm not sure any tube will work with any kind of circuit anyway.

You might install let's say three small tubes to form U if that looks more contemporary.