Finding a teaching position in Ontario has become very challenging in recent years.  You may want to read the results of the most recent Transition to Teaching survey (http://www.oct.ca/publications/professionally_speaking/march_2010/features/transition.aspx), which tracks the rates of employment success among Ontario teacher graduates.

You can find extensive information regarding the process of applying for a teaching job in Ontario on the employment section
(http://www.teachinontario.ca/employment/En/1_getting_hired.html) of the Teach In Ontario website referenced above.  Although primarily developed to assist internationally educated teachers navigate the Ontario job search process, Teach in Ontario includes excellent advice that can be used by teachers prepared here in Ontario too, including a section on working as an occasional teacher.
Many school boards in Ontario rely on employment websites to advertise and fill teaching openings.  The four main sites that are commonly used are:

• Education Canada (http://www.educationcanada.com/)
• Jobs in Education (http://www.jobsineducation.com/)
• Workopolis (http://www.workopolis.ca)
• Apply to Education (http://www.applytoeducation.com/)