If you buy posters from the retail stores across Toronto, they are bound to be expensive. Framing your posters would further increase the price. Just like you, I also searched around the city and couldn't find what I wanted. So I went online to search and found http://www.allposters.ca/ . I was reluctant to try at first but for most of the posters, they were selling for quite reasonable prices as compared to buying from retail stores. So I tried purchasing some nature posters of forest, waterfalls, island beaches and waterways.
The posters arrived in very good condition and the quality of the posters was quite impressive. The paper used on the posters were durable as well. I would suggest putting a big cello tape along the entire edges of the posters. This is to allow you to put more cello tape to attach poster to the wall if you are not framing your posters. In the future if you do decide you wanna move the posters to a new location on the wall, you can safely do so without damaging the actual posters paper when you rip the cello tape off the wall.
Allposters also sell framed posters but for a bigger price. Although I never had the desire to frame my posters, I can't say how good or bad the quality of the framing is.