As of Wuthering Waves Version 1.1, characters who are using Loong's Pearl as Ascension materials are Jinhsi. You can find this ascension mat in a small location in Loong's Rest, Mt. Firmament.
There are also many Loong's Pearl that can be found in an underground cave and you can only enter this place after progressing in Main Quests [Jinzhou Rising, Huanglong I - Act 7].
All 65 Loong's Pearl in Wuthering Waves
You can find the first 50 Loong's Pearl in the marked location above. One flower can be plucked twice so you see why there are only 25 flowers on the open-world. In addition, most of them are placed near together so just use Sensor to highlight all the nearby resources that can be farmed. And of course, you can also refer to the video on top for the location of each flower.
Remember that the x26 Loong's Pearl can be found in an underground cave and you can only enter this place after progressing in Main Quests [Jinzhou Rising, Huanglong I - Act 7].
For the last 15, you can buy it from Koko in Shifang Pharmacy, Jinzhou. You'll need a total of 45,000 Shell Credits to buy all of them.