I'm sure someone will correct me if I'm wrong, but to my knowledge APs are not sold in Canada. Not all Disney stores carry them - definitely call first to check. If you need to access the passholders section of disneyworld.com (passholders section of the site allows you to book the special discounted room only rates, and gives you info about special events and merchandise for AP holders), then it's probably simplest to order online through the Disney site. When you purchase the AP online, what they send you is an Exchange Certificate which has a code on it that will allow you to access the website.
You turn in the Exchange Certificate at Guest Services when you go to WDW. The AP is valid for one year from the date you get the actual AP.
Here is my tip to save a little - and you want to purchase in advance - if, for example there is going to be a price increase, or just want to spread out the cash outlay - but you *don't* need the passholders website access...purchase the AP online and select the 'will call' option instead of having them ship you the exchange certificate. With this option, you print out your own receipt at home. When you go to WDW, you bring that receipt to Guest Services (it has a bar code on it), and exchange that for the AP, exactly the same as with the Exchange Certificate.
FYI when you turn in your receipt (or Exchange Certificate), they will not charge you any increase in price between the time when you purchased and when you exchange. IIRC the shipping was ~ $20 for my Exchange Certificate a while ago (it is shipped UPS or DHL, not by regular mail). I got my DH and DS their APs using the 'Will call' option just before a price increase.
Every little bit counts!