It is possible to borrow (at no cost) a wheelchair from your resort. Generally they have them available, but they are first-come, first-served, and do not take reservations. They will take a credit card imprint and you return the chair at the end of your stay. But they are heavy and, as all the Disney wheelchairs, do not have adjustable foot rests.
If you click on where it says "disABILTIES" at the top of this page it will take you to the Index. The top item is the "disABILTIES FAQ" which will gives lots of useful information, including where people rent wheelchairs from off-site. These tend to be lighter in weight, have adjustable footrests, and can be a better size for each person.
Wheelchairs can be rented at the Gift Shop at TTC and the Outpost Gift Shop in front of AK. Otherwise rental would be inside the parks.