I know exactly what you are looking for and replaced a couple about 6 years ago. Got them at the dealer. The parts guy you tried at the dealership either looked at the wrong diagrams - sometimes the name they give for a part doesn't make sense because the translation from German to English is wrong.
On Genuine Mercedes Parts
they list a couple of items on the diagram (see below) that are called jack cover fastener and a couple of other items related to the jack cover. The cover basically is a friction fit to the rocker cover so there isn't a need for a rivet/etc. I would bet this is the item you are looking for. Does the dealer order parts and if they are wrong would they still charge you - mine doesn't.
When I log on to my other computer I have an old diagram with part numbers. I check it out and see if they have something different or at least get you the part number for the parts I found on Genuine Mercedes Parts
. At least it will be a start - check again with the dealer. 2000 MERCEDES-BENZ CLK430 Parts - Genuine Mercedes Parts