Philanemo mushrooms are a plant in Genshin Impact that you will need to ascend Klee, Barbara, and Mona. It is an awkward one to find, as it only grows in a couple of places. You can find these mushrooms in the city of Mondstadt, and the town of Springvale.
It is not that simple, however, as they tend to grow on the walls and roofs of houses. The easiest way to find them is to simply fast travel to either location, then use your Elemental Vision to easily highlight where they are. This way, the plants will be easily visible to you, and you can climb up and get them.
Springvale, despite being smaller, is the best place to farm them. You should be able to get about 10 here every day if you need them, and there are fewer buildings to check. After that, you should head to Mondstadt, but they can be harder to track down in the larger city.
Keeping ahead of your needs is important in Genshin Impact, so you can just quickly grab a few of them every day, that way you will always have enough when you need them.
If you need some very quickly, you can visit Chloris on the path to the east of Windrise. She will sell you some Philanemo Mushrooms for 1000 Mora each, but will only be able to sell you five of them every three days.