W220 Rear SAM Replacement

So I have an S280 and the rear SAM needs replacing.

1) Do I need to get a specific S280 SAM or can I get any Petrol W220 SAM? There are lots of S320 W220 SAM's for £100 on Ebay, will they fit okay?
2)Apart from EBAY does anyone have any other suggestions of where to buy from?

Any advice gratefully received!