ZTTO Internal Frame Cable Damper Foam Sleeve 6mm 1.5m

The reviewer does not discuss cable insulation as a distinct bike part. The only related mention is the use of electrical tape (Isolierband) as a makeshift fix: he wraps a bit of tape around the brake line and the internal routing connector so they stay together and slide through the frame as one piece, and he also tapes the brake line to the fork to prevent rattling.