The p/n beginning with "210," 2104000002, is indeed the MB
part number, and "210" means the part is for Mercedes model
W210, which is the current E-class.  I'm not totally up to
speed on this model as we have previous W124, but am pretty
certain they all come equipped (in USA) with 16-in alloy
wheels and therefore this wheel is probably most often
procured from dealer for snow tires.  I wouldn't be
surprised if Tire Rack or Discount Tire can alternately
supply these wheels.  I don't think many will be found in
junk yards.