The one depicted in Texholdem's post is pretty much the only mass-produced cup holder I've seen produced for the W124...and there are a number of sources for it.

Beyond that, you can custom fab one yourself. Personally, I stuff my drinks in the console, and slide the door against the cup to hold it.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car