I usually go to Regal Plastics. Great prices and they are easy to work with.
I have made a few fairings but I use 1/4" ABS. I have had them on vehicles for 10 years or more with zero issues. still they clean up and look as good as the day I made them.
ABS is usually black, cuts great without chipping, sands great to soften the edges, flexes well to get a curve, has a textured side and a smooth side (I use the texture side out) and it's durable as anything.