Image via miHoYo
Loach Pearl is a very rare resource in Genshin Impact that can only be gotten from Golden Loach, a type of lizard that can be found in Liyue. You will normally only find one of these lizards out in the wild in any given location, but they can appear in groups as well, so prepare to be quick on your feet.
On the images below you will see various area maps, with the Loach Pearl location shown by the player indicator. You will need to be quick, as these guys can move quite fast. Just grab them as you would any other animal or flower in the game.