I'm sure you have Zehrs or Zehrs Superstore in Toronto and the GTA... I recall seeing pizza stones there in the $10-$12 range. I also believe that IKEA has them. All else fails, open up the yellow pages and look up the list of kitchen/restaurant supply stores in the GTA, I'm sure there are many!
I purchased my last one at a restaurant supply store here in Windsor about 5 years ago; it's 35cm square and cost roughly $10. BTW, I do suggest you get a square one rather than a round one, it gives you more surface area and more leeway when you slide the pizza on and off the heated stone. A pizza stone is also great for baking bread, and I can easily fit two loaves on my square stone.
Many people also recommend purchasing a large unglazed tile from a building or flooring centre, but I've yet to find one that sells tiles large enough here in Windsor, perhaps in the GTA you'd have better luck.
Hope this helps a bit.