I've had good results with this discount car rental broker for Maui.
http://discounthawaiicarrental.com/

Here is another similar one, though I haven't used them.
http://www.carrentalinhawaii.com/

In my experience, they provide a contract through one of the major brands, but are able to get cheaper rates than generally available. Once you make the reservation with them, all future dealing is directly with the car rental company.