Their website says "We are unable to accept reservations for these items. All wheelchairs, scooters, strollers, and lockers are available are available on a first come/first serve basis."
You can call them, but it's first come first serve I'm almost positive...
I don't know if you can in advance, but they do have them there. You will have to pay to rent, and get them from guest services right inside the gate.
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