FIRST find your chassis/vin number-- then ascertain whether you have a straight six M104 engine or a V6 M112 --- then go to this site and search for the part nos you need Modifications MERCEDES SL 320 [Car] [CHASSIS] (EUROPA)
then enter the numbers into a search engine to get an idea of availability/price
then come back.
Mercedes parts if available will be pricy. OEM/ aftermarket may be available but likely to be middle and rear section only. Front sections may be harder to source. At this point you may want to contemplate a custom St/st exhaust as an option?