I recently bought the driver's threshold cover for my '71 280SE. The part # is 108 686 0180 7003. 7003 is the color code, and you need to include that number for these covers to show as available on Mercedes parts websites. 7003 is for the very dark gray that would appear to match your car.

I was also surprised at the expense of new W108 bumper rubber -- there was a steep price increase just a few months ago. With patience you will probably find good used ones at used MB parts places or eBay, where I bought mine.

Bill in Maryland