There are supercharger kits available from either:
http://www.tdi-plc.com/Merc500SEC.SELclass.html
Or if your feeling more extravagant
http://www.koenig-specials.com/mercedes/mercedes.html

I'm sure there are more kits available, but these are the ones I know of.

Kind Regards

__________________
Ben

'85 500 SEL
'85 928S2
'88 560 SEC
'90 750i