Nissan does not publish a towing capacity for the Leaf and the owner's manual states you should not tow with this vehicle, so we would suggest using the Kia for any towing duties. You can visit a Kia dealer for recommended tow bars, but we would suggest trying an independent specialist, ideally one that also offers a fitting service so you can ensure the tow bar is correctly wired into the vehicle's electrical system. There are numerous online retailers who can help you, and respected brands such as Witter, Westfalia and Truckman are good choices.