The cruiselines all have preferred vendors for medical supplies needed for your trip. Whether it's a wheelchair, walker, oxygen or whatever. The cruiseline doesn't provide these items except for pier assistance which is free, just have your T/A notify the line you need a wheelchair for embarkation or disembarkation and they'll arrange that for you as a complimentary service.
The two companies most cruiseline have on their recommendation list are as follows.
Cruise Ship Assist 1(866)889-8099
Care Vacation 1(877)478-7827
Both these companies rent the medical items and coordinate with the cruise line to have it delivered to the ship and picked up when you return from your cruise. You can also bring your own if you have it and wish to. Just check with the airlines about any restrictions or notifications required and notify the cruiseline that you'll be bringing it.