I check what is available first on http://matrix.itasoftware.com
Then I use the same dates and either book through GCR for $18 cash back, then enter www.cheapoair.ca.
I also book through www.skyscanner.net since the prices are sometimes cheaper than matrix. I also recommend http://www.google.ca/flights/ and www.expedia.ca
Check for any online promo codes.
I pay with either my MBNA World Travel MC card or MBNA Smartcash MC for additional money back.