Not available for everyday purchase at the ticket booths.
Occasionally a one day hopper, often with an expiration date, is created for a promotion such as by time share condo. sales outfits.
If you have an old multi-park WDW ticket such as a super duper pass, if there is just one day left on it, you would do an even no-money exchange for a modern ticket that fits the fastpass machines and what you get in that case is essentially a one day hopper.
One day hoppers have been given out as rainchecks in the past, when some emergency requiring closure of the park occurred.