Front wheel(s)? Rear?
I had some trouble finding these as well, though I didn't exactly kill myself looking...
At $52 a pop, Suncoast (
http://www.****************************************************.com/product/986flap&Category_Code=POTEBXEX.html) has what they call the rear "wheel arch trim".
I also had some injury sustained to the front wheel counterparts and was also actually missing some stuff up there I didn't even know existed, specifically all or part of "flap 2" and "flap 3" in the pic at the following link:
http://www.renntech.org/forums/index.php?/topic/1111-boxsterboxster-s-mud-guards/page__fromsearch__1
I
think (but am not positive) the following links (right and left side, respectively) describe all or part of the missing front wheel pieces (ie "flap 1", "flap 2", and/or "flap 3" in the previous RennTech pic).
http://www.pelicanparts.com/cgi-bin/smart/more_info.cgi?pn=996-504-604-00-M100&catalog_description=Air%20Guide%20for%20Fender%20L iner%2C%20Right%20Front%2C%20986%20Boxster%20S%20% 28through%202004%29
http://www.pelicanparts.com/cgi-bin/smart/more_info.cgi?pn=996-504-603-00-M100&catalog_description=Air%20Guide%20for%20Fender%20L iner%2C%20Left%20Front%2C%20986%20Boxster%20S%20%2 8through%202004%29
If you (or anyone else out there) can figure it out, let me know. Otherwise, I guess I'll go to the dealer with the RennTech pic and the car, find out the part numbers and prices, decide they want too much, and order them elsewhere.
(Or maybe I'll just call Pelican and/or Suncoast.)