Another vote for Brevard Medical Supply.
If you are looking at a standard wheelchair, you might consider whether it will be used at the airport, etc. It can be cheaper to buy one second hand (think Goodwill stores) than to rent one. There is no charge for it on the plane.
DCL has some standard and large sized wheelchairs for complimentary use (usually found by the midship elevators on decks 1 and 2) as well as some kept in the medical center for accidents, etc. There are beach wheelchairs on Castaway Cay. All DCL chairs are "first come, first served," so you can't assure that there will be one for your family member.