Anyone have any advice for me as I have a bad cat converter on my 2002 SL500? I understand Mercedes has discontinued manufacturing replacement parts. Any suggestions would be appreciated.

 

Adsit has aftermarket ones. They sell about 4-5 a month for R129 owners.

 

Are you looking at replacing the entire system? ( This is basically the manifolds housing the primary cats and also the mid pipes housing the secondary cats )
If you're only thinking about replacing the secondary ones, then Magnflow make them and they are very good quality.
If you are thinking of replacing the entire system ( specifically the primary ones ), then there's a company in the UK "BM Catalysts" that make them. I have done this. But I must warn you, the cheap prices reflect their quality. But, they work.