I can't help you with the "where" part, but a head's up regarding the "what" you are looking for. Most common North American sizes used are #6 by 48 threads per inch (6-48), and occasionally the larger #8 by 40 threads (8-40). These come in various lengths - say 1/4" on up through 1/2" or more by 1/16". Not to be confused with the screws on the rings themselves - for example, Ruger uses #6-40 screws on their rings. Often find a tray or box with all kinds of strays - mixture of everything - may or may not be what you need. Some scope base makers (like Weaver) are set up for a screw with a taper under the head; others like Parker Hale have a squared off underside on their scope base screws. Might be an idea to actually remove the screws that you want to replace and take them with you when you go looking.
I don't know what Winnipeg has left for "gun shops", but I would think any place that installs scopes would have a collection of loose scope base screws. You will also want to be using hollow ground or parallel side screw drivers that fit tightly in the screw slots - a tapered screwdriver, like from Canadian Tire, will bend and distort the screw slots because they only apply pressure on the very top edge of the slot, and very seldom fit tightly.
