I rented from the Dolphin a week or so ago, through Alamo. It was about $85 for a three day rental for a Pontiac G6, but they gave me a Buick Rainier instead (same price). I would go to the Alamo or National official sites and start plugging in dates. You can find some decent on-line specials on the sites also.
The only car rental agencies at the Dolphin are National and Alamo. I've found that making advance reservations thru these companies will USUALLY yield the cheapest rates using various codes and coupons - the best place to find these codes/coupons are on Mary's site, mousesavers.com