In other cases, non-microwavable plastic wrap works too.
A latex barrier may also be called a "dental dam".
More About Latex Barriers
A latex barrier is designed for oral sex. Stretched to fit over the labia or anus, it is meant to decrease the risk of exposure to sexually transmitted infections.
If a latex barrier is unavailable, a condom for a penis can be cut to cover the anal area to create a makeshift latex barrier.