You can get them at Highland Farms. But I find the quality so-so. They tend to be short and can break. I made 20lbs of sausage this past weekend, so am particularly sensitive to poor quality casing. I went to my old standby, Jerry's Supermarket on Danforth between Coxwell and Greenwood. Long casings that don't break too easily.
If that's not convenient for you, I'd check any reputable butcher that makes sausage in house.
850 Ellesmere Rd, Toronto, ON M1P, CA