While many things are cheaper in Ecuador than in the US, that is not the case for a rental car. Per the other comments, go to your usual websites like orbits, expedia, etc or directly to the large rental car sites themselves. Expect to pay a bit more than usual for a car. Also, since you are a foreigner, the rental company will require you to take the daily insurance on the car for an additional $15-$20 per day or they will put a hold on your credit card for roughly $5,000, just in case there is an accident. The flip side is that gas is cheaper there.
Totally agree on the driving w/o a GPS. It forced us to get to know Ecuador a bit more than if we were blindly taken from place to place. Lots of fun and little stress but totally worth it. Have fun.
