I don't know about the airlines but in the past we rented car seats and booster seats with our rental car directly through the agency (National, Alamo, etc.) I think I paid $15 for the week per seat in addition to the car rate but that was several years ago. For us it was worth it not to have to bring ours with us. If you don't need it for the flight check with the rental car agency to see if it's an option.