I bought mine from an Ebay seller - admittedly for my S203, but it fits my S204 well enough. It's washable and has zips either side of the front piece so you can lay that flat when you drop the rear seat. Sellers ID was Araches, but I can't now find him. There are plenty of car boot liner sellers on Ebay.
Thanks guys, will have a look. Its to contain a shaggy dog. I have a Mercedes boot mat that did a great job for the floor of the old car but need something to try to protect the sides and back of seats from the hair or at least try to minimise the accumulation of it.
Suppose the other option is to him from getting in the car!!