Hi there Annie,
Welcome to planDisney and thank you so much for stopping by with your question. I'm happy to hear that you are planning a visit to the Happiest Place on Earth later this fall! It looks like your visit would be in the midst of the
Halloween season, which is one of my favorite times of year to visit the parks.
There are
electric conveyance vehicles (ECVs) for rent at the Stroller Shop just outside the theme parks, though they are available on a first come, first served basis as you mentioned. When I have visited the parks with family members who required the use of a mobility device, we ended up reserving one through ScooterBug, which is Disney's Featured Provider.
They offer reserved scooter rentals for a similar daily rate and they can drop the rental off at Bell Services if you're staying at a Disneyland Resort hotel. It's worth a look if you need a bit of extra peace of mind with a confirmed reservation for your ECV during your visit.
Additionally, the
Accessibility hub on the Disneyland Resource has a wealth of additional resources to help you plan for your trip including information about experiencing attractions, disability parking, and more.
Thanks again for your question, Annie! Make sure to come back and see us if you think of any follow-ups - we'd love to help.
Have a Magical Day,
Greg