If you are looking for a Royal, you *might* be able to find one on TUG or the owner's manual - omcancun.com .
I believe ISCO and VCR do rent for just one night but only as you are getting very close to the date. You can also look on the TUG BBS under last minute rentals.
When we needed an extra night we rented at the Omni through Travelocity. It was about $160? for the room, including an excellent brunch buffet for all four of us. The Holiday Inn was less expensive, around $135 with a brunch, but the Omni had the advantage of being able to walk right next door to the Tri-Royals.