Are you talking about the square flat caps that cover the jack holes on the sides of the car in the lower cladding, above the rubber jacking points underneath the car?
If so, these are easily obtainable from MB and are cheap. I ordered I think 6 of them about 5 years ago and they came to me painted in the correct lower cladding color.
The part number is 124 698 13 30 and the plastic strap is 201 698 02 14. Four of each part are required for the car. You will need to specify the color code -- if you have a 199 (pearl black) car that would be 7700.
Cheers,
Gerry