The single button ones exist, I bought one just dig around a bit. A complete unit from MB is ~ $ 150 ( can't get the housing ) you will need to provide title / owners card and your picture ID. Some MB dealers need to see the car also through I don't think this is MB policy.

I try to buy non China but they seem to be the only ones willing to put an effort forth to make key programmers , key housings and the like. I won't buy china bearings or something I can get in the US.