I often find good deals at www.autorentals.com, autoslash,www.economybookings.com as well as priceline.com (use bid feature), hotwire.com and carrentalexpress.com. Don’t forget to ask for promo deals, affiliation discounts, e.g., (aaa/aarp) and “local” only deals. I recently found good deals on craigslist.com. Most recently I needed to rent a car for a week in Columbus, Ohio with drop off in Detroit and got a rate of $700 from hotwire and priceline but it only cost me $300 with economybooking. go figger!
Consider, check out the new personal car rental services at airports such as flightcar.com or transporting a car via a service like https://www.autodriveaway.com.