It is often called "corrogated plastic". It is sometimes used as temporary shelter, I found mine at Homedepot near the doors section, right beside large sheets of plexi glass.
Correx is another name for it, however I am not sure if that is a US brand or not.
Call sign shops, hobby shops, craft stores, and hardware stores. Tell them you want Corrogated Plastic to build a bottom for a guinea pig cage. Often Sign shops don't want to sell you coroplast thinking you are making your own signs, but are more then willing to help if you tell them you want it for a small animal cage.
