Intake Air Control Motor?
 

My 1988 560SL idles high and inconsistent. I thought that meant a vacuum leak, but was told it is likely a bad "idle air control motor", which I was shown is located on top of the engine near the front. However, I cannot find this item listed in the PeachParts categories and if I search for this part by name, I don't get a match or suggestions. Does this part go by another name?

MB calls it "Slide"/ Idle speed actuator. PN 000 141 1625. AKA air slide valve. idle control valve.
You could have problem with the Idle control relay. The solder joints fail with age. PN 006 545 8532

Peach Parts calls it an Idle Control Valve with part number W0133-1604260.

However, before buying the valve...
Your first guess of unmetered air or vacuum leak is very possible and warrants checking out.
Also the Valve can be cleaned with choke cleaner, often allowing it to work correctly without replacement.