If you are looking for seat belts, here is a shot of my 121 interior, showing the original lap belts (likely dealer installed) from 1966. I think that they are similar in style to the "green" ones you have been seeking for your coupe. Like your sister car, my 230SL's interior is original, but the carpet was replaced when the original owner gave up driving at age 85 and was offering it for sale in 1989.
Check with your a local dealerships and find out who their used car departments use for interior repairs. A good interior guy can do wonders with the tears in the seats and may be able to re-color the dash coverings as well. Square weave carpet may be more difficult, but, again, I've seen interior guys perform some pretty amazing magic on trashed interiors in my 40 years in the car business!
