Canada has a repertory of all properties on sale on the Multiple Listing Service (MLS) which is kept up to date by real estate agents. Some home owners like to sell their houses without the help of a real estate agent, but they are few.
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 .