As I don't own a car either, I had to do this often when renting. You can purchase liability insurance by itself from the car rental agency. Most are $10-15 a day. It's usually termed SLI or something similar. Just decline the other stuff like CDW that is covered by your credit card and agree to the SLI. This must typically be done at the desk rather than when reserving online. About a year ago, I called up Geico and bought a non owners liability policy for $40 a month that covers me no matter the vehicle I'm driving. Good luck.