A list of postal codes for cities in Ontario can be found at GeoNames.org. GeoNames provides a search tool to find postal codes by country, province and city.
To find a list of postal codes for Ontario, navigate to GeoNames.org and click Postal Codes. From the list of countries, click Postal Codes Canada, then click Ontario. The resulting page lists postal codes for cities in Ontario along with a link to a map of the area.
GeoNames provides links to find postal codes for different countries throughout the world. The website also provides a reverse look-up link that lists cities with a given postal code.