Ontario G1 practice tests are available online, some for free and some with a fee attached. One good resource is the G1 Canada site, which offers multiple G1 practice tests for free in both 40-question and 100-question formats. The tests are in an online format, so you take them directly on the website and receive the results immediately. The site provides several tests that include changes made to the Ontario G1 exam in 2015.
An advantage to the G1 Canada site is that in addition to providing practice exams, it also offers tips and information regarding the driver's license process and what Ontario requires of new drivers. The site covers everything from documentation to what to do if you fail the G1 exam.
The G1 Driving site also offers practice tests, and allows you to select the number of questions from five to 50, then take the test directly on their site. The practice exams are free, and the site has an inventory of over 200 questions.
Another good resource is Ontario's official transportation website. The site provides official information about the testing process and offers a study guide for the written test for a fee. The site provides instructional videos for both G1 and G2 exam candidates and has a thorough FAQ section.