The best source to find vintage
RPGs in Toronto is "The Sword & Board" located near Bloor & Dufferin. I haven't been there in quite some time, but the last time I was there, they had a whole wall of photocopier boxes full of used
RPG items. I found about a half dozen shrinkwrapped AD&D modules during my visit. I only went through their boxes labelled D&D, but they had a few dozen other boxes of used
RPG items, so it's possible you could find something of value if you devote the time to search through them. My visit was about 4 years ago before the
RPG market became "hot", so it's possible everything has been picked clean, but definitely worth a look.
Another potential source of items is "The Hairy Tarantula" on Yonge just south of York Mills. It's a mix of
RPGs and collectable card games, however, this store used to be heavily into
RPGs back in 80's/90's. I first went there in 2019 hoping to find some vintage D&D items but, unfortunately, it was all modern merchandise. However, back then if you asked a staff member about their old inventory, they would escort you into the "off limits" store basement to browse. I remember there were literally stacks of books and boxes everywhere. I bought some great vintage items that day at an extremely good deal. The owner of the store is a bit of an eccentric though, and when I went back there a few months later, I was told that the owner had forbidden access to the basement even to store staff, so I was out of luck. I would love to get a chance to browse through everything as there was a TON of vintage stuff down there. The Hairy Tarantula also has a store on eBay where they sell some used vintage items and even in their listings descriptions they admit to "...we have a plethora of inventory that is out of print. If you are looking for a rare, or hard to find game, or comic book, you have come to the right place. Anything we don't have currently as an active listing, please feel free to email us and we can look over the hundreds of thousands of items we have in stock to see if we can make it available and at what price we would be selling it for." The hoard is real, but unfortunately, inaccessible to browse anymore.
Good luck on your searches!
Richard