Have you searched Fastlane... I am pretty sure they would have one.
http://catalog.peachparts.com/ShopByVehicle.epc?q=&yearid=1990%40%401990&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6236%3AMBC|1520%3AED|10000014%3ASM|850%40%40300TE+4Matic&keyword=valve&subcatid=P:241573@@Suspension+Valve&mode=PA

Have it shipped to someplace just across the border (if you are close like i am) and go pick it up.
J.