I don't think the rubber covers have never been sold seperatly. These were not made by Mercedes or installed at the factory, but were installed after the cars were shipped to the USA either at the port of entry or at the USA dearship where the car was sold. When we ordered new bumper guards back in the 70s the replacements did not come in Mercedes packaging and were only sold as a set of 4. They were made for MBUSA by a subcontractor.
You might want to check with a USA based vendor that makes reproductions (K&K?) and ask if they sell just the rubbers.