I have just had part of my GTI front bumper resprayed due to next door having a slight coming together with it whilst parking and bought the black clip-on trim from the main Citroen people here in Exeter, with clips.
Three day delivery from "head office", £32 for 2,(one each side).
If this is the same sort of trim you are searching for, only for the rear, try your main agent.
JohnHA.