There are a variety of online sources to aid in your search for a rental apartment in Ottawa. Kijiji and Craigslist Ottawa are two of the most popular.
Kijiji is a large classified ad site that's very popular in Canada. As of November 2015, it has more than 4,500 listings under the section labeled Apartments/Condos. You can filter and sort the listings by price, size, furnished or unfurnished, and distance from a chosen point in the city.
Craigslist is an online classified ad site that started in San Francisco and has since spread to hundreds of cities worldwide, including Ottawa. It too has thousands of listings under the Apartments/Housing for Rent section of the site. You can filter the listings by neighborhood, rental price, number of bedrooms and bathrooms, pet policies, and other terms.