A dental dam, also called an oral dam, is a square piece of thin latex or polyurethane, the same that's used to make male condoms. It's placed over a woman's vulva and/or anus during oral sex to prevent the spread of STIs. They get their name from their use during dental procedures to keep teeth dry (my dentist totally uses these).
They come in a variety of sizes and flavors, but in order for them to work, they must be used consistently and correctly. They're really easy to use even though they may seem a little awkward at first. Just open the package, rinse it off under water (they come with a powdery talc on them), and you're good to go. The person performing oral sex just holds the dam over his or her partner's private parts and that's it, you're safe and protected. The receiving person can apply a water-based lubricant on their vulva or anus, and this should help keep the dam in place and increase sensations.
This may be obvious, but when you use a dental dam, only use one side. Don't flip it over as this can transmit fluids, and never reuse a dental dam.
Where can you find them? Some drug stores may carry dental dams, but your best bet is to order them online. Sheer Glyde Dams are approved by the FDA and come in Wildberry, Vanilla, or Strawberry flavors. Buy them in packs of four ($5.95) or packs of 12 ($12.95). This may not be the most glamorous thing to keep in your nightstand drawer, but anything that keeps you protected against STIs gets a thumbs up in my book.