While there are a couple you can find in the desert area near misty coast and south of camp overwatch. That said the most Scarlet Thornes i was able to find were in Tiderise Cliff. You will be able to find around 14 Scarletthorns here
Wuthering Waves Interactive Map. Find locations for ascension materials, ores, supply chests, tidal heritage, echoes, collectibles, sonance casket, side quests and much more.