When looking for Budget car rental coupons, the first place you may want to check is
They are currently* offering a weekly car rental coupon for $20 off a weekly rental.
*at the time this question was being researched and answered.
Budget car rental online coupon codes are also available at various coupon sites on the web. You can check out Free Rental Car Coupon Codes for a list of some of the most popular sites for online car rental coupons. Also, check for offers and deals if you are checking rates on sites like Travelocity, Expedia, Priceline, etc.
Budget rental car coupons are also available in the Entertainment Book for most cities in the United States.
Hope this helps!