http://www.memoryexpress.com for buying drives and sometimes graphics cards although I like to shop around to get the best prices. They are local. I build my own PCs so have only bought components from them but I see they also sell systems.
http://www.amazon.ca for cases and drives (I check pricing first on camelcamelcamel.ca to make sure it’s the cheapest possible in price history)
+1 for memory express. They also price beat any Canadian retailer by 10% the difference I think - both online and brick&mortar. I’ve been using them for years as they were the Alberta equivalent to NCIX before they expanded into BC.