i'm not sure if it's worth looking for existing fairings unless they're designed for your car
often these are part of the wheel well inner wall or the attachements where desined for a speciffic car.
find a big flat piece of plastic and perhap some L shaped brackets and you should be able to cut something that fits perfectly and that's setup to aid the airflow over the speciffic car structure. you could probably get a fairly large sheet of black abs plastic that you could make several different models form to test.
this autospeed article has some info that might be usefull but it's not that complete that it can be used as an exact guide
http://autospeed.com/cms/A_2456/article.html
there's little further i can find on them on the web, not even a general name.
so i suppose i'll have to keep sticking my head under parked cars when i seen these things a look at wich parts of the wheels and suspention it covers and perhaps measure the distance to the ground and the center of the weel etc...