I just bought mine from an online sign supply company. The only catch is they can't ship it in anything larger than 2'x4' sheets. The 6mm that I got looks like it will do the job just fine once I finally get a chance to get under the car.
With the election year coming up, there are TONS of free coroplast to be harvested from every intersection corner. Just take down the person your not voting for's campaign sign.