I think it is called Correx (or Corex) as well - so if you can't find Coroplast in the US try searching for Corex. I got 10 sheets of A1 Corex on ebay uk for £29.95 which included the postage. They are huge and would make a pen half the size of our living room! They arrived within two days, well packaged as well. Here's the link for the seller I bought from:
The link is for White Corex even though it shows lots of different colours, it does say white: Must get round to making ours!
10 sheets of A1 fluted plastic corex board for out door signage and display | eBay