Our local Dollarama - largest dollar store chain in Canada - has a sheet protector (of all sizes) that works wonderfully for this purpose. Amazingly, not a drop of the unwanted bodily fluids seep through to the matress, tho I would not want to try leaving the plastic on for longer than a day! YY has peed the bed twice, one when she was caught under the blankets and needed to go and another when my 6 yo niece was playing with her in the bed - and both were "supposed" to be asleep. My niece thought YY wanted to leave her and YY just really really needed to go.Both times, this plastic came to the rescue.
You could prob find a similar product where you are. There are kitty diapers or you can make your own the but those are typically for kitties who have serious and/or chronic kidney problems.
For some reason, this reminds that the first Dollar Store I ever saw was in that New Orleans Mall that is on the river. (sighhhhhh!!)