Technically, they're "litter box liners". But the purpose is the same, to absorb liquid. We got a package of 20 off Amazon that are the exact same size as the base of the carriers, which is great for the older cat. The younger cat didn't like it as much as the soft, fuzzy cat bed, and shoved it into a bunch at one end of the carrier. But it was there if she needed it during the flight.