This topic has been discussed ad naseum many times. Buses and Taxis do no commonly have nor are required to have car seats and booster seats.
You will have to call and wait longer for a car that has one... at WDW that may be as long as 45-60 min.
If you personally want a booster or car seat suggest you bring it and take it around with you. But you won't find seat belts on buses and they can be difficult to locate or may be broken in taxis.