It depends on how much you are willing to spend art stores sell foam core by the sheet depending on the type and thickness it could run $40 for a 4'x8' sheet.
But Walmart sells smaller bits in there school supply area for science displays that work quite well for buildings. And if you are not picky model railroad shops have buildings that can work as well then there is this place
http://www.davesgames.net/wfrp2/ hope that helps.