Hello Pierre-Etienne,
welcome at Stridaforum 
I do not think that single tubes are available, but feel free to ask directly at Strida.nl.
But I also believe that an exchange of the bottom tube is not necessary in most cases, it should be possible to repair it, in worst case by exchanging the plastic insert 100-07 and installation of an „armour“ shim like shown below:
Unfortunately, the TriLock system is sold out right now, so I’d recommend to try first either my complete „brass“ system or a mix of it with some standard parts.
Kind regards,
Chris