The disABILITIES Baord is coming to you.
Here's a list of the places that DIS posters report renting from most often:
Care Medical:
http://www.caremedicalequipment.com/
Phone (407) 856-2273 Toll Free U.S and Canada (800) 741-2282
Walker Mobility:
1-888-726-6837
www.walkermobility.com
RANDY'S Mobility is in Kissimmee 407-892-4777
http://randysmobility.com/
Colonial Medical
http://www.colonialmed.com/about_cms.html
(800)747-0246
All of them will pick up and deliver to your resort. Randy's requires that you be there for pick up and delivery. The others will drop it off at bell services.
You also have the option of renting each day at the parks, but those are first come, first served and if you are there later later in the morning or park hop, they are often all gone.
You can follow the link in my signature to the disABILITIES Board fro more info.