I would suggest picking up your local newspaper and looking for a section that lists the garage sales in your area. If your paper doesn't have one, try your local
Craigslist's garage sale ads. Then just go to as many of those as you can.
There is also the
Retro Game Store Project which lists game stores that specialize in classic games, but it seems to be USA only. There might be something similar for Canada though.
Or, you could just browse the
Sales Extravaganza board and look for stuff you want, or post a wanted ad if you're looking for something in particular.