WAIT!! dont buy the expensive walmart cavas. Go to your hardware store (menards for me) and buy a cotton paint drop cloth and some white wash. Works just like canvas. Get a coupla two by twos to strech it and your set. plus when you strech it, you get like a istant frame.
http://www.kissig.com/Stretchcanvas.html If you get wood cut to be a frame you can just use staples in the corners.
Heres picture of what i mean:
White wash $5 Trap $12 (17 total) VS $7 a yard ($30 for the size of the tarp.)