I thought you guys might find these sites useful. They're used book sites where they have listings from various used bookstores around the world.
http://ww.alibris.com
http://www.abebooks.com
How these sites work is they provide listings from the various used bookstores that have signed up with them. When you order things through their site, they send the order to the store you chose to buy from. The deal itself is between you and the store.
I've used abebooks several times and have not yet been disappointed. I've never used alibris, but I've heard good things.
*Remember:
SHIPPING TIMES:
It takes time for these books to be shipped. If you want them for the beginning of the semester, you need to order now.
SHIPPING COSTS:
Pay close attention to the shipping costs. Make sure you look at the price to ship to Canada. Sometimes these costs can wipe out the savings.
CURRENCY CONVERSION:
Make sure you pay attention to how much it'll cost you in Canadian dollars.
BUYER BEWARE:
Like anything else you buy on the Internet, you can get ripped off. So use these sites at your own risk. If you have problems, you have to deal with the stores themselves, not with alibris or abebooks.