I have seen rentals on eBay.
The "Last Call" that we got (which is dirt cheap) is a cash rental thru RCI that can only be made 6 weeks or less from your arrival date. We were actually looking for something on the west coast of Florida, but this was the best we could get on that short notice. It's been fine.
[ETA: if you know someone who is an RCI member, a Last Call is doable. You will have to pay an extra guest charge as noted above (which may vary depending on the timeshare your friend owns), but it will still be a great deal if you are comfortable waiting that late. Orlando is a very overbuilt area for timeshares and there are always Last Calls available somewhere. We had a choice of 5-6 resorts, most of them RCI Gold Crown resorts.]