Where would one look to find farms that were around in the 40' + 50's? Ont Fed Agri doesn't have that info.

It depends what you are looking for. If you're looking for a list of all farms in Ontario, you're out of luck. If you're looking for a specific farm and know the approximate area where it may be located, check the city directories of surrounding areas, check land records, and most of all check the libraries in the area for a history of that township. Some genealogy societies have compiled land histories that document whom has lived on what plot of land and when. Most of these publications are available in local libraries or through the genealogy society that published them.