Achim,
This all started me in the direction with this thread
https://www.sl113.org/forums/index.php?topic=30415.0 You may have seen it. So what peaked my interest was the fact that bright red interiors should have the medium grey mats. I have a '60 220SE W128 with such an interior color and had bought black sill mats (which is what I am selling) but recently found medium grey. Yahoo!
So that should be correct for that car. My W111 has cognac which should have dark brown which is not available that I have seen. Too bad because that would look nice. I use black for that and is OK. You are looking for dark grey that Aaron mentioned and I have not seen that. But to answer your question, the following people I found have the medium grey mat and I am sure there are others - Authentic Classics and Mbclassics.de. So these are not what you are looking for if you need dark grey. Sorry.
If you know somebody that carries the dark brown mats, I would be very grateful.
Wallace