The Guinea Pig Zone - Where the Passionate Piggy People Go!
Plastic Land
Hot315 Church Street
New York, 10013
(212) 925-6376
http://www.ttplasticland.com
Map
User reviews
It's about $20 for a 4' x 8' sheet of Coroplast. There's a good selection of colors available. Take a few minutes for them to bring it up from the basement but they have it in stock. If you want more selection Canal Plastics Center is just one block away (but is about $2 more per sheet).
Good luck finding parking for either location; there are one hour spots of both sides of Church street but you'll be doing a lot of circling before you get one.