Try the Capricorn Bambu Lab AMS kit. The tubing is 3mm ID / 4mm OD and is 4m long.
https://forum.creality.com/t/k2-cfs-modifications-the-extruder-and-cfs-tubes-connections/29603
I found it here in the USA:
https://www.captubes.com/shop/#!/Bambu-Lab-AMS-Kit/p/593740313