AAA and Kayak. I have a car booked with each. Alamo on both. AAA was $550 for an Audi sedan and Kayak an Audi SUV for $500, six nights. Standard SUV's were high $400's. I tend to book, find a better deal and then cancel until the trip.
www.autoslash.com is the best for publicly available rates. If you don't have one Costco membership can pay for itself with a single rental. Same with AARP. I pay the $12/year for AARP just for the discounts. AAA if you already have it, rarely does it save enough to be worth paying the membership fee just for the rental.