Former mortician here. These aren't used everywhere or all the time. If your family requests your body be embalmed, all the liquids and semi-solids of your insides will be sucked out of your guts using a hollow spear hooked up to a vacuum. If done right, there should be no liquid left in your body to leak out.
The funeral homes I worked at didn't have these. If it was necessary to plug the anus, we'd pack it with a bunch of kapok fiber. It's like cotton but doesn't absorb liquid.