There is a guy on mercedesclub website called charliecarnut or something like that, he has several w111 coupes that he is stripping for parts. What you are looking for is not too difficult to find. You can find then on ebay also, but stay away from atvm and another guy who I can not remember at the moment. I like to test sellers some times by asking them questions and see how they respond. Some of the common ebay coupe sellers are selling items like retractable seatbelts for early w111's, but when you comment that the early w111 can not accept retractables, they say that they can all you have to do is cut the sheet metal under the passenger side and then re-sew the rear seat upholstery..... yeah anything is possible, but not everything is resaleable. The point here is that your parts are easily available but you need to be cautious of who you are dealing with.
If you need pictures or help email me at leonardbrenner@sbcglobal.net as I personally redid my leather upholstery last winter and know those seats very well.
Again what is the year your working with? The early w111 are a bit different then the later w111's
Leonard