Do you need to put in a reservation for a wheelchair at Disneyland?

Nope! Just go up to the wheelchair rentals on the left from the main entrance to the park and you'll be able to rent one. Last time we rented one, they didn't even ask for a doctor's note. Even though it's kind of pricey, I recommend renting an ECV, pushing people around Disneyland all day can slow you down and tire you out.