Where can i buy a big sheet of canvas to make a banner?

Where can i buy a sheet/canvas to make a banner - i need it cheap as is for muck up day at skl - and just wanna write some stuff - so some plain canvas but very big... Anybody know?

Buy Galaxy Products Canvas Drop Cloth here http://digitsy.com/us/search/Galaxy%20Products%20Canvas%20Drop%20Cloth/page1