Whoops! I edited my post so the coro and aluminum wouldn't be on the same line...coro is not available from LOWES (sorry for the mis-understanding!)...yes it is corrugated plastic sign board, and you should be able to find it in the yellow pages by looking for sign supply shops. It's also what gas station type cigarette and cola signs are made from. It is also available mail order in many different colors...lots of links on the Spad site under supplies in the hints and tips section.
http://mywebpages.comcast.net/TipsTr.../tip_index.htm