Rob,
Just did this last year. I think your best shot is ebay, but you may need to be patient. When you say showroom condition, I'm not sure what you mean. Your other one will have some road wear, so you will probably have to match it. This means finding a used one. I regularly find "headlight doors" on ebay. If you are replacing both sides, New Old Stock (NOS) will cost you $1200 per side. I'm not sure what Mercedes charges from the parts dept, but you may try to contact one of our fine vendors on this site. If you put a new one on one side, with a "buffed" one on the other, it will look weird.
If you want to do used, there are two ways to go. One is the complete assembly with the door, chrome, and turn signal modules attached. I was lucky enough to find the plastic door only, which I could fit my electronics into. Of course, the complete assemblies are more expensive. I think I paid about $350 landed for my door.
Hope this helps.
Mike Mizesko
Columbus, OH