Loomis was cheaper than the 5 art stores I went to in TO and Brampton. You know the kind, tons of oil&arcylic paintings that all look the same. They wanted $13 an 8x10 board (extra if I wanted a window cut) with a price break at 500 boards (only at one store). I didn't bother asking about larger sizes.
You could try Micheals but I think you'll find the price compareable, if not slightly higher to Loomis.
There was an art store on Queen St. east (north side of street) near Kew Beach. Don't know if they are still there, but their prices seemed reasonable last time I was in there. Sorry don't remember the name. On the south side of Queen in the same area is an art dealer/gallery called something Collector (Incureable perhaps). They do framing, might be worth while to inquire there as well. Worst comes to worst, you'll get to look at some nice art.