240DBenzer,
I had the same problem with my 83' 300 cd. The drivers seat was sagging on the back left side. I priced a new spring box from that Worlduphostry place, $250.00.
I think that the coupe seat springs are harder to come by. I settled with swapping the drivers with the passenger side. It took me about 6 hours to do it, but I took my time and gave the interior a good cleaning while the seats where out of the car. The drivers seat is a whole lot more comfortable and the passengers seat sag is hardly noticeable.
Marks88
83' 300 cdt
88' 300 e