There are a few places where you could look when you are ready to buy a dirt bike. Make sure you choose a dirt bike that's right for you.
You should obviously start with trader.ca and kijiji.ca++.
However, there are lots of deals to be had at the major dirt bike dealers and shops around Toronto. Most of them will put their used inventory online, so it's easy to check.
Here is the list of Dirt bike shops in Ontario. And don't forget to read about how to check used bikes, here.