Spares for Mercedes Roof Bars for ML270 (W163)

[MarkBeattie]

Does anyone know where I can get a set of the plastic cups (that come with the Mercedes roof bars) that sit on top of the roof rails that hold the roof bars in place ? I have a set of roof bars, but the plastic cups are missing (hope I have explained this well enough !!). Mercedes dealer told me they do not supply these seperately ?? Thanks.