Radio knobs, where from?

Hello All.
Just bought a 58 plate polo diesel with the basic RCD200 i think it is.
The cap out of the end of the on/off knob is missing, any ideas where i can get one from.
Main agent says "you have to buy the complete radio and seeing as you have a knob missing theres a surcharge". Nice.
Anyone any ideas.
Cheers
Paul

Ebay or a breakers yard seem like your options if a main dealer can't supply one.