I guess this is a reapeat but I am going to leave it.
https://www.yelp.com/biz/sun-valley-mercedes-transmissions-llc-van-nuys-2
I bought used rear trailing arms from Sun Valley Mercedes but they are also know for their rebuilt transmissions. A couple of our members have mentioned them and even gotten advice from the one of their transmission guys.
The best way to be sure you are getting a used Engine or Transmission is to test drive a body damaged car that is selling cheap and if they are OK buy the whole car.