I posted pics of my 228i M Performance diffuser several weeks ago (see url below). I painted all but the fins body colour.
The part number for the 228i M Performance rear diffuser is 51192343354.
The part number for the 235i M Performance rear diffuser is 51192343355.
Ergo, you want part #: 51192343354.
You should be able to order it from your friendly local BMW dealer - or from any number of online sources.
http://www.2addicts.com/forums/showthread.php?t=1134171