You can rent one from Care Medical - they have a better assortment than if you borrow from the resort. They are also more likely to be in good condition. Follow the link in my signature to the disABILITIES FAQs thread and look in post 2 for rental information.
If you borrow one from the resort, as others have said, there is no guarantee that one will be available. I would suggest that you check it out carefully before using it. We've borrowed one twice send found the crews were loose both times - the second one was very wobbly. Since most people don't carry screwdrivers on vacation, that's something you want to check out before you are all ready to take your shower or bath.