We got our rental from the airport in Hilo since that is where we flew into for our stay on the Big Island. There is also an airport in Kailua-Kona area, Kona International Airport at Keahole (KOA. Pretty much all car companies are represented at both airports.
Driving from Hilo to Kona took us about 4 hrs.
There are a lot of cars driving about in Kona but it is fairly easy getting around. There are ample parking spots.
It is highly recommended to spend the extra money for a convertible rental car. To fully experience The Big Island, it is great having the top down to hear the birds, smell the flowers and feel the warm sun. Sure, it costs more, but is well worth the extra money.
This is the BIG Island. to see or do anything outside of your hotel or resorts immeadate area you must have a rental car. And be patient. Roads are narrow and winding on most parts of the island. It's gonna take you awhile to get anywhere. But hey, your' in paradise-- whats your hurry?