RE: Corrugated plastic sheeting
You can get all you want for FREE!
All the Political signs you see are generally made of this material. Just pull over and grab a bunch. Can't get into trouble because the signs are give ups. Well, not if they are in someone's yard.
The lettering will peel off with a bit of effort, or just let it be.
You can always ask your local sign shop to save the scraps.