There are wheelchairs available at all the resorts, but there is no guarantee that one will be available at any resort when you check in.
Here's what Disney has to say in their Guidebook for Guests with Disabilities about wheelchairs at the resorts:
Wheelchairs are available in limited numbers for Walt Disney World® Resort Guests at each Resort. Contact Guest Services or the Front Desk for assistance. A $315 deposit will be held on the Guest room account charged only if the wheelchair is not returned. Guests wishing to guarantee the use of a wheelchair throughout their stay should contact local area rental companies to make arrangements.
There are some links in the disABILITIES FAQs thread to outside rental places that DIS posters have used and been pleased with.