A set of four genuine blue and chrome W126 axle caps, with 'MERCEDES-BENZ' with wreath on the circumference, and central MB star will set you back about $140 retail, from Marshalls Motors, Parramatta.
I didn't buy. I had mine colour coded for free instead, with the star remaining in silver. ( I have 15" W126 wheels on my vehicle. The matching spare wheel came from a junk yard.)
If you need only a single axle cap, head for a junk yard. It shouldn't cost you any more than five bucks. When W116's were easier to find in junk yards, I was able to source quite a large range of body parts.
There is probably a dozen or more significant items that now live again, on my car.