McMaster Carr has a good selection of plastics or just about anything else you can think of. Site is also pretty easy to use.
BLUF: expensive unless you are making more than several sets to sell to recoup the cost. www.alumilite.com - I've used them for other auto parts with great success. similar to smooth on. there are other threads on here about this. trick is getting the color right for colored lenses and no bubbles for clear.