As the lone Canuck dropping in here for you ... most of the suggestions won't help you much, unless you go State side to shop.
I'm from the other end of the province (West of GTA) and would head to a Len's Mill Store. So if you're up this way for some reason, then that'd be the place to head. It often has ends of lines, and sell offs ... though still good quality.
Sorry, not familiar in your area as to what you have for possibilities. Fabricland would carry it, if you have one nearby. Somewhere that carries upholstery fabric, specifically outdoor quality is where you need to go.