Ensuring a car rental reservation is held i.e. that the agency has a car for your when you arrive

I recently booked a car through a price aggregator site. After booking, I immediately received an email stating a reservation confirmation number. When I arrived at the rental agency, they said they had no car for me and to call the price aggregator site. I did. They told me that there was no guarantee that the car would be held and that I would have to go elsewhere to book the car.

I haven’t read the fine print, but in the immediate FAQs and text of the email, no where does it say that the car will be not held.

I don’t want to have to read lengthy terms and conditions every time I book a car. Is there a sure fire, yet quick way to make sure that my car is held when I reserve it?