i agree with poofie, i would get the motorized scooter if you can afford it.
two options, either rent in the park or from an outside vendor.
we usually get them in the parks- and to answer your original question- yes they are easy to get. you cannot get them until you are through the park gates (except at internatinal gateway at epcot) so it does slow you down with the rope drop rush- but ony a few minutes. wheelchairs are the same story.
we like to rent them in the park because then we don't have to transport them back and forth ourselves, as we always rent a car. but it is not a problem at all if you are taking busses. and it is indeed cheaper to rent off site.
if you do rent in the parks, the hotels have free wheelchairs for you to use while there. so we drop my 80+ year old parents off at the hotel door, and get the wheel chair from bell services, and wheel them up to the room. you can keep it in your room the whole time you are there. when we are leaving, we drop it off at bell services and they hold it for us until our return.
if you do not want to spend the extra, yes, wheelchairs are cheap and easy to get. just not easy to push all day as poofie said, the parks have lots of hills and a lot of ground to cover