Have you checked your local Home Depot, Lowes or Agway? Other than that you can check out these links:
SOLEXX Corrugated/Insulated Plastic Panels:
http://fishstoretn.com/corrugated_in..._plastic_.html
EnviroCept Company:
http://www.greenhouses-etc.net/glazi...e_covering.htm
Insulated polycarbonate panels:
http://www.google.com/search?hl=en&q...=Google+Search
To me they seem expensive, I'd think about using some doubled up poly tarps at 4 mils thickness. A 10'x25' (4 mils) is available at Home Depot for less than $14.00 per package.
Robin