Find a car rental agent as opposed to one that owns it fleetThey usually use the big boys cars at discounted rates just to get cars on the road.Their terms and conditions are usually different.

http://www.carhire3000.com/Home.do?affiliateCode=google&label=Brand1-2YgrkRB4QlxIfecm*UlDgQS9558746652&ws=&gclid=COO54MLm9LICFVMbtAodQjcAcQ

I got one from Pisa airport,took me a few minutes of head scratching and found they where using Herz cars at half the price.