Flight centre is great because you can pay in Canadian dollars, so you don't have to worry about exchange rates or extra credit card fees for foreign currency. You can also get them through Westjet orAir Canada if you're flying with them, but they will likely be more expensive than Flight Centre.
We bought these last summer and it was a great deal. Basically you get one park free. We also upgraded our DL passes to 4-day hoppers when we got there.