Someone on the Atari forums said he likes Monarch Computer and orders from Canada. The only thing I can say is the last time I looked at their shipping cost, it wasn't that bad.
Compuvest also ships to Canada but their shipping is more expensive. Tigerdirect.ca also ships and you won't even see any duty on your package, just the provincial tax on your credit card. But I'm sure they add the duty on their web site. Despite the negative reviews about Tigerdirect, all you have to do is do your research on the components you want to order before ordering from them. And if there is a rebate on something, follow all the instructions to the letter or you won't get your refund. Also, a lot of components with rebates on them are just cheap no-name brand crap that will give you problems.
pricenetwork.ca
pricegrabber.ca