We did this last trip as my mom had had hip replacement surgery 5 months earlier and it was just too much.
Your best bet is to get one from your resort and then you will always have it and its FREE from the resort. When we requested one at ASMo they had to get one from ASSp and they were nice. We had one incident with a rude CM who said they don't reserve them for guests and she was wrong since we had one with our name on it and after 10 min of telling my Mom was wrong, she fiannly asked for my mom's name and looked in back and Lo and Behold- there it was.