Long ago, many of the resorts had a small stash of wheelchairs for guests to use in an emergency (we borrowed one for my mother). But I don't think they have them any more.
I had to rent a wheelchair for my wife this year for our entire visit. I contacted Buena Vista Rentals and they delivered to the resort and picked up when I was done. it was only 12 dollars each day. The resorts no longer have wheelchairs, at least that is what I was told when making our reservations.