Well sadly a number of the stores in the Toronto/Missisissauga area have closed.
Depending on if you have a vehicle, there is an awesome store in Bolton Ontario called One Crazy Stamper. It would be about 45 minutes to an hour from Mississauga/Oakville but it's a great store with great selection of stamps (check out their on line web site and you'll see).
There is one store in Mississauga called Scrapbook Queens that has some stamping stuff but alot of scrapbooking.
In Markham (north east end of Toronto) there is a place called Pointer Scrapbooking that has a decent selection of stamping stuff too.
I live just west of Oakville but my favourite store is the Memory Keeper in Hamilton. They have a ton of stamping stuff/nestabilities etc. Great prices, great selection and easy access from the highway.
Hope this helps. If you need any more info let me know