We had good luck with Cobblestone Garden a few years ago when we planted Swedish Aspens. I haven't been there this year so I don't know what their inventory looks like. It's located on 17th Avenue between Calgary and Chestermere.
http://cobblestonegarden.ca/
Also, Home Depot has a sale right now as well. It's a buy 2 get one free deal. Here's the link to the coupon. It expires on the 14th though.
http://www.homedepot.ca/wcsstore/Hom...D%20COUPON||||