at Costco.com. That is for Alamo in Orlando anyway. What you need to do is go to Costco.com (don't need to be a costco member BTW) and click on travel. Then click on rental cars. Then look for whatever "coupons" or "deals" they have on the left hand side. If there is a deal there that interests you, click on it then it will guide you through the rest.
I've checked too many times to count on how to get the lowest rate, and I've found nothing lower than costco.com