Unless someone has a spare, or you ask around at breakers yards, they are difficult to find especially the AMG version.
Maybe one of our supporting vendors stock them? If not try here...
AMG Tow Hook Cover unpainted medium gray w/ black mesh screen
WA1298800405, right $355.28 w/ S&H (passenger side)
WA1298800305, left $355.28 w/ S&H from Parts.com
You will have to get it/them painted to match the colour of your car though!
EDIT: When you find a replacement, remember to fit it with a cable tie or a very small screw. Most of us R129 owners do this!