Ah, but even though you don't have a JoAnn's in Canada, they do have a website with 40% off coupons, so you could order online.
I usually get the 200 count white muslin that is either 90" or 108" wide (I can't remember, and they probably have both), because I'm lazy and don't like to piece backs. At the store, it's about $8 a yard, so the discount coupon would probably more than make up for the shipping charges.
There are several other websites where you can get 108" backing that is either printed or in solid colors. It will cost a bit more, but is worth looking into.