Manual wheelchair and Electric Convenience Vehicles rentals are available inside the Main Entrance for a fee of $10.00 for manual wheelchairs and $35.00 for ECVs. Both require a refundable deposit of $20.00.
If you're going to be there for longer, or you will need the wheelchair outside the parks, it might be better to check into renting from a medical supply company. The disAbilities board is more WDW-oriented, but I believe they have some companies listed if you need them.