Look up www.mercedes-benz.com/amg/
There should be contact info for AMG in the US. I contacted AMG in Canada and they emailed me a list of everything they had for my 1986 420SEL (W126 like yours) They have exhaust systems, areodynamics kits, complete suspension kits, spring kits, steering wheel, rims and AMG badges. I don't think you could order AMG S-Class cars from the dealer like you can get ML55 ad E55 and te S55 today. The list they sent me was pretty expensive.
Hope that helps