I've had good luck locating used parts on
www.car-part.com.
You can search the entire database in just a few minutes... not sure they have "mirror" as a selection criteria, but they certainly have interior parts, etc.... do a search for your year/car, find out where the cars are, and then call the yards that say they have that car...
Every yard I've dealt with so far has guaranteed their parts to work when they arrive...
Only takes a few minutes to change a mirror in a W140.