There may be some wheelchairs available to borrow at the resort, but if you really plan to use one regularly at the resort and parks I recommend renting from offsite. The ones at the resort are basically there for emergency needs, may not be in good condition, may not be the right size, etc. You'll be better off renting from offsite, and it's usually less expensive than the in-park rentals that must be returned before you leave the park each day. As PP indicated, the FAQ sticky above has information about renting.
Enjoy your vacation!