This is a
link to the disABILITIES FAQs thread. That's where you will find information about all the places to rent
ECVs (electric scooters). There is information about renting from the parks and also links and phone numbers from the companies that DIS posters have pretty consistently reported good experiences with. Many people rent one for their whole stay because the resorts are large and there is a lot of walking involved just in getting from place to place.
If you are only renting an
ECV for a day, you probably would be best off renting at the park you are going to be going to. BUT, there is no guarantee that they will have one when you arrive, but usually if you are there early enough in the day it won't be a problem. If they don't have any ECVs, you will be able to get a wheelchair.
Just a clarification - ECVs (motorized scooters) are different than power (motorized) wheelchairs. ECVs are steered with a 'tiller', sort of like steering a bicycle.
Power wheelchairs are steered with a joystick.
You can rent ECVs from the parks and also from off-site rental companies (like Randys). Power wheelchairs can
only be rented from one of the off-site companies and they will
only rent them to people who already have experience driving one (for example, my DD uses a power wheelchair at home, but has not traveled with it).
If what she needs is a power wheelchair, you will need to find out the model and settings on her wheelchair before you go so that the company can try to make things as similar as possible.