Welcome to disABILITIES.
All of the frequent DIS posters who use
ecvs must be busy today, so I'll give you some information that will be helpful. My DD has her own wheelchair, so we've never rented an
ecv, but I can give you some general information.
The time period that you are going is not likely to be busy, but there is no guarantee that an ecv will be available at the park you are hopping to. If worse comes to worse, there should not be any problem getting a wheelchair at the second park; they are not as limited in quantity as ecvs are.
Here are the outside places that DIS posters report renting from:
Care Medical:
http://www.caremedicalequipment.com/
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
Besides not having to worry about getting one at the parks, anothere advantage is having an ecv for getting around your resort and for places like DD that don't rent ecvs. If you are staying at a WDW resort, almost all the buses and boats and all the monorails are accessible.