07-21-2011, 12:24 PM
I've posted elsewhere that for large, basic sheets, you may be able to get them used from your local transit authority. I have managed to get a couple of the 2x6 sheets used for bus-side advertising. You may need to peel off vinyl printing from some of them. They make great building "carcasses", being (usually) 60 mil or better. They are also good for painting, as six months on the side of a bus makes sure there is no shiny-ness left... 
Andrew
Andrew