What about trying one of the Niederlassung (Mercedes Super Store) dealers such as this one in Hamburg.

Maybe you could try and buy directly from a dealer in Germany.

I think they have Niederlassungs in Berlin, Munich, Frankfurt and elsewhere

I believe this place sells out of its parts Department on www.ebay.de (Germany) and ships worldwide - which included to USA I can't find the ebay link to them though Most places have people that speak English

http://www.starcarvigator.com/index1.php?lang=1

Berlin
http://www.berlin.mercedes-benz.de/content/germany/retailer-1/niederlassung_berlin/de/rep_InternetVertriebsCenter/For_International_Customers.html

Munich - parts

http://translate.google.com/translate?hl=en&sl=de&u=http://www.muenchen.mercedes-benz.de/content/germany/retailer-8/niederlassung_muenchen/de/home/passenger_cars/home/parts_and_accessories/parts/original-tauschteile0.html&sa=X&oi=translate&resnum=1&ct=result&prev=/search%3Fq%3Dmercedes%2Bbenz%2Bniederlassung%2Bmunchen%2Bparts%26num%3D30%26hl%3Den%26sa%3DG

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1991 560 SEC AMG, 199k <---- 300 hp 10:1 ECE euro HV ...

1995 E 420, 170k "The Red Plum" (sold)

2015 BMW 535i xdrive awd Stage 1 DINAN, 6k, <----364 hp

1967 Mercury Cougar, 49k

2013 Jaguar XF, 20k <----340 hp Supercharged, All Wheel Drive (sold)