Advertising on bulletin boards is a good idea; and probably free or dirt cheap
It crossed my mind that there maybe some opportunity with seniors who own pets, but who aren't quite as mobile and active as they used to be. Perhaps there's a seniors' council in the area. It might be worthwhile checking in on them. There seems to be demand for the service in my home town. And it looks like there's good bucks to be made. This is one of them ... neat website too:
Kingston Pet Sitting Services : Home - Kingston, Ontario, Canada