Rent one from off-site.
Partly for the reason Eeyore Tattoo says, and partly because if you get to a park, and discover that you need one, there is no guarantee that they will have one.
Go with an ECV - pushing a wheelchair is TOUGH at
WDW - it is NOT level, anywhere AND if you are being pushed, you are at the mercy of the pusher as to where you go and what you see.