Papalo, also called Papaloquelite (porophyllum ruderale or macrocephalum), is an herb, similar to cilantro used in Mexican cooking. The name comes from the word papalotl, Nahuatl, for butterfly. In Spanish, the plant is referred to as mampuitu, which translates to skunk. It is typically used raw to flavor tacos filled with guacamole or carnitas (pork). It is also used in a Puebla specialty using a semita roll and stuffed with meat, avocado, tomatoes, and sometimes chiles. Like cilantro, the herb is quite pungent and is an acquired taste. You can read even more about papalo here.
You probably won't find this outside of a good Mexican market or produce market. If this is a herb you enjoy, you can grow your own. You can purchase the seeds at Johnny's Seeds. The plant is very pretty, grows up to 4 feet tall and likes sun but will tolerate some shade.
If you can't find papalo and don't care to grow your own, then you can substitute: