Tom, as a 40 kWh owner, I agree that you will likely be very hard pressed to find a seller. First, us 40 owners already have upgrade potential with the car we have, since our cars have a 60 kWh battery pack with a software limitation. So, we can pay Tesla down the road to unlock the additional capacity if needed. Second, we had three chances to upgrade during the configuration process (when we first configured, when they decided not to make the 40 kWh battery pack, and again after we received our cars when Tesla sent an e-mail giving us an option to upgrade at the original cost differential), so everyone who has a 40 has already thought long and hard about their need for an upgrade. I would suspect that anyone who still has a 40 (me included) is using it exactly as you plan, as a city car.
Is there a reason you don't want to buy a new 60? Actually, just as I was typing that, I realized that you are in Canada, so you may be looking to buy used from the States such as not to pay the "Canadian premium." Makes sense.