I've always found Costco's codes to be among the most generous. They're easily found by going to Costco.com and selecting Travel. They usually also have a working link that will take you directly to each Car Rental company's site with the code pre-filled, or at least some of the session information so that you may not even have to manually enter the code. I know what you're thinking.. you may not be a Costco member. While I do think being one is worthwhile, there is no Membership required to visit the site, and I've never been asked for an ID to this date, while visiting WDW 3-5 times a year.
Good luck!
George