If you're looking for a used bike, there are three picks on where to find what you're looking for in Canada.
First of all, Facebook Marketplace has more and more decent options for bike shopping. What I like about Facebook Marketplace is that you can see who's selling the item, ask them questions, and check to see that they're legit. Secondly, Kijiji is always a decent go to for used in Canada. And thirdly is Pinkbike.com, which is more of a legit cyclist/mountain biker's scene.
Kijiji and Facebook differ from Pinkbike in that Pinkbike is generally populated by more experienced riders. So you'll often find people who know the ins and outs of their bikes a lot better, and you'll usually find higher quality bikes on there priced somewhat fairly. On the flip side, Kijiji and Facebook can be all over the map sometimes with hilariously overpriced "Canadian Tire Specials" and suspiciously underpriced diamonds in the rough. Whenever I list my bikes I always list them on all three, but Kijiji and Facebook are a bit of a free for all when it comes to pricing and the quality of bikes.
Now if you're looking for a new bike, I strongly recommend your local bike shop ("LBS"). I work casually at Ridley's Cycle in Calgary and they've got a fantastic team there that'll get you on the right bike. If you want to set yourself up for success, have an idea of what kind of riding you plan to do, and have an idea of your budget, and be prepared to spend another $100-$200 on things like helmets, pedals, bike pump, etc.