Cheapest place to get Keurig cups and the best selection is from Keurig. They ship to Canada as well and it is free shipping on orders over $45.
http://www.keurig.com/
They are starting up a Canadian site, but I don't think you can register there yet.
Costco might have cheap Kcups but the selection is basically non existent.
Cayne's Housewares now sells Kcups as well and has a fairly good selection. They started carrying the large size boxes too, so 48 to 54 Kcups in a box.
They also have free shipping on orders over $100 and other free shipping specials throughout the year.