Not in downtown, but it might be on your way to town
Birlington Camera is an honest independent camera store, run by the same family for nearly 50 years. They always have a good selection of good used cameras. They don't buy cheap used lenses, etc.
The basement is well stocked with new and used darkroom and lighting equipment. The display cases on the main floor have a displayed collection (yes, the cameras are for sale) of almost every circa 1930-1980 roll film and 35mm cameras that you ever are likely to run across from most manufacturers .
The staff are all competent, and there are always one or two on the floor that are very knowlegeable and patient.
I am not associated with the store, but am a loyal customer. It is about the same time and distance for me to go there vs downtown Toronto, and most of the time I opt for Burlington.
They are located 2km south of the QEW on Guleph Line, on the west side, just north of New Street. Take exit 103 off the QEW, which is after you come over the Burlington Bay Skyway, and pass the Highway 407 exit. This presumes that you are driving in via Niagara Falls. If you come by way of the 401, then you will need a road atlas to get yourself better organized.
I second the Henrys central store recommendation for downtown. It is huge, but there are areas for everything, including used film cameras. they have an Ebay auction, and Henrys.com web site that will gibve you a flavour of what they have on offer. Further west on Queen Street is Vistek, but they have more a pro/rental focus than older cameras, but do have some better representation of meduim format cameras that they sell used.
If you are sure about what you want, and know the product, there are a host of pawnbrokers round the Henrys main location that move more mainstream used cameras, but you must know what to look for with a broker sold camera.
Mississauga has Just Cameras, off the south side of Dundas (a plaza that fronts onto Blundell), just west of Dixie. Mostly a new retailer, but some used stock. Henrys Mississuaga store is at Hurontario and Eglinton on the north east corner. Mostly a new camera retailer, but some used.