Can anyone please tell me where I can buy the Height Control Module for my GL450 on 2007 Mercedes-Benz GL450

I did a diagnostic test on my GL450 and was told the Height Control Module is faulty because my car sinks at the left rear shocks when I park and rises when I start which had also happened to the front shocks before.

Asked by for the 2007 Mercedes-Benz GL450
i think that is dealer item only or go good used from wrecking yard