You can find 10 years' worth online at CRA site
Need to find out the status of your income tax refund? What about review prior years' tax returns and carry foward information? Can't find your T4 or T4A slips for last year? Good news - all this information can be found online using the Canada Revenue Agency's "My Account for Individuals." To use the full system and get access to all of the above information and more, you need to register. To do so, you need to obtain a CRA user ID and password, which can be applied for online. You first provide some personal information, create a CRA user ID and password, and create security questions and answers. You will then have to wait for a security code, which will be mailed to you from the CRA, which you enter to gain full access. Once you're registered, you can view your past 10 years' of tax returns online, along with lots of other handy information.