Renting an
ECV (
scooter) at the parks is $45 a day plus a $100 deposit. There is no way to garantee an ECV ahead of time and they do run out of rental
ECVs fairly early some days (in the busiest season it can be within an hour of park opening). Wheelchair rentals at the park are substantially less expensive and don't tend to run out, but that does require her to have someone with her to push as you really can't self-propel the park wheelchairs for any real distance.
If your MIL is going to go to WDW, or other places that require a large amount of walking, often she may want to look into purchasing her own scooter. There are several scooters out there that break down into small enough pieces to fit in most cars with the heaviest piece being around 35lbs. Depending on her health insurance coverage, she may be able to get a prescription from her doctor so that the insurance company will pay for the scooter.
One of the stickies on the top of the disAbilities board is a FAQ. You should read through it as it will actually answer many of your questions.