They don't sell lenses for our cars only the whole headlight assembly and they are expensive.
I suggest making them look brand new with the tips described in this thread.

However, I believe the most cost effective solution is to take your car to Sam's Club and have them do it.
They
1) Wet Sand your headlights
2) Polish your headlights
3) Apply several coats of clear coat with a UV-filter to avoid future fading (without that, you will be back to square one in less than 6 months)
4) You receive a 5 year warranty on scratching, fading, clouding.

All that for $25! Couldn't buy the kit for cheaper than that! Had mine done almost a year ago and the lenses look like brand new. Makes a huge difference in the appearance of the car!