Sorry, don't know of any, but the car rental companies often have promotional coupons in many magazines (especially travel related ones) or special rates in certain regions. Try a internet search on "car rental discount" and you should find some possibilities.
If you work for a major company, it is likely the rental outfits offer them a corporate discount. Ask your employer or the rental agencies - usually any employee qualifies. Unions, credit unions and other groups may also have discount arrangements. So do AAA and AARP, if you belong to either of those.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL