I have no idea where he got it from, heck he might have even made it himself, but kcpaz has been doing a Subaru STi engine conversion project on a Boxster track car, and he has one hell of a spoiler going on out back. If you go a few pages in the pictures will start to show, and you'll see the spoiler he has (it's pretty extreme). I'd contact him to see where he got his.
Here's the thread:
http://986forum.com/forums/general-discussions/25697-boxster-subaru-conversion-interest.html