Where to find the Calathuia plant? This is actually easier to find than one may think; the Calathuia can be found inside the main river (Noob River) down in the tropical area. You are going to have to swim for it so bring plenty of breathing potions. I always bring a sickle with me because you get more bang for your buck that way and faster.
Look for this pink plant along the river as you swim:
One it’s in your inventory, it will look like a blue and white flower.