If you sign up for the newsletter on mousesavers.com, it will include a link for undercover tourist tickets at a greater discount. The newsletter comes out mid-month so you have time to sign up for the May addition. The prices are in US dollars but include the taxes and shipping to Canada.
If you have CAA, you may want to check if they have any specials.
Mousesavers is a good website to start your research.
Another vote for UT. I've ordered from them 4 or 5 times without issue. They ship from Toronto, so there is no customs issues. Remember to compare the full prices after tax and shipping (and US$ Conversion if applicable). The CAA discount is not stellar by any means.