If you want to travel freely in Sado Island, car rental is recommended.
Here are some companies that you can rent a car in Sado Island.
TOYOTA Rent a Car
Address：353, Minato, Ryotsu, Sado city, 952-0014
TEL：0259-27-2100
Open：Mon to Sun 8:00A.M.-6:30P.M.
Times Car Rental
Address：144, Minato, Ryotsu, Sado city, 952-0014
TEL：0259-24-7211
Open：Mon to Sun 8:00A.M.～6:00P.M.
JR Rent-A-Car
Address：353, Minato, Ryotsu, Sado city, 952-0014
TEL：0259-24-7050
There are other car rentals, but their website is only in Japanese, so these above would be more easy to make reservation for you.
Car rental get booked up quickly during peak season such as Golden Week(from the end of Apr to the beginning of May), Summer holidays(from 20th Jul to the end of Aug), the end of Sep, triathlon event and Long Ride event on the island, so I recommend you make reservation early.
