Yo make it yourself. A splitter would be one of the easiest body parts to make for a car, for cheap too. Go buy a 1/8 in sheet of balsa (or plywood if you only want looks) cut it to the shape you want and then glass over it. You could use real Carbon Fiber if you want to be a bad ass (takes a bit more skill) or use reqular resin and glass and mix in graphite. I know it sounds like a cheesy ricer thing to do to get the black color, but its actually sold by west systems as a low friction addative to be added for creating rudders keels and skegs in the boating industry. Unlike teflon which isonly a coating this will not wear off. If you want any other ideas or tips let me know, cause there is so much bad ass stuff you could do. I mean theres foam core instead of balsa, and you could use kevlar, or uni or bi cloth and all sorts of stuff. Components for mounting would also fairly easy to come by, just takes a bit of creativity and a good hardware store. As for aerodynamic shape, take a look at someone elses but a plane symetric foil mounted with negitive incidence would be the easiest way to go. Composites rule by the way. Im torn between Carbon Fiber and nomex comb for my favorite.