For a property to be listed on the MLS, the seller must sign a contract with a realtor. This has the advantage that all Canadian real estate agents know that the property is available for sale.
If you would like to do some research before asking a realtor, you can find information on websites such as Realtor or Househunting .
Although websites give you an idea of the market, they can not advise you like a professional can. The realtors know their way around, can help you find your dream house (as they have access to more listings than you) and can even help you negotiate a better deal.
If you would like to do everything yourself, you can read local newspapers with classified sections, such as The Montreal Gazette , The Toronto Star , or The Vancouver Sun .