I think what you want is a scooter (also called ecv). Unless you already drive an electric wheelchair, none of the companies will rent one to you. I don't know the price of an ecv from the outside places (I'm sure someone who does know will post). The price from the parks is a $10.00 deposit + $30.00 rental Here is the contact info for the outside rental places:
Care Medical: http://www.caremedicalequipment.com/