As far as I know there isn't a mass produced one. All have been custom jobs. Of course without a smooth bottom on the car, a diffuser does precisely dick other than show people how much you want to be noticed.
A car does not need to be "skinned" underneath for a rear diffuser to be functional , but it greatly helps (see the Evo). However, I don't believe anyone will spend the money in wind tunnel time to design one for a wagon with a little over 100HP.
pretty much what everyone else said, doesn't do anything performance wise it's purely an aesthetic mod. You'll need to make brackets to get it mounted. there's also the voltex replica that shi fu jay installed on his silver p5
