Hi Lori!
I am glad you are addressing your health needs now versus arriving and not being prepared. Every guest is anxious to arrive at the parks and be ready to hit the magic head on.
Wheelchairs are sometimes sitting near the bus drop off areas but that is never a given. You just can't depend on that happening since park equipment is not allowed to leave the park. There is quite a bit of distance between the bus area and the wheelchair/ECV rental area for all four parks.
Please consider renting from an off site company so that you will have either a wheelchair or an ECV the entire time of your stay. All of the Disney buses are prepared to handle with the loading and unloading of these vehicles so your transportation is made easy and stress free.
Check out the recommended
Rental Companies that Disney recommends. These will be places that will deliver to your resort so you have the item there when you arrive and will also pick it up when you depart. It is very easy and they will make sure you are aware of the instructions in operating and keeping them charged.
Renting vehicles at the parks is also easy but I would not want you to suffer trying to get to the location.