We ave not used any in Orlando, but used Wheelchair Getaways (which has a branch in Orlando) when an accident took our van out of commission for a week.
Where ever you rent from, they are expensive. I don't think you will be able to find any that are based out of the Orlando airport.
There is wheelchair accessible transportation from the airport to hotels (Mears, Yellow Cab and DME) and if you are staying at one of the WDW reorts, the WDW transportation in accessible.