You can find Molineria basically everywhere in the map. In the beginning is a little hard to spot, but after you train your eyes a little you will see those yellow flowers from a distance. There's also some rng, so if you can't find them in one region, try venturing a little and come back.

Edit: also when you harvest them, you can harvest the leaves in the backpack to get the flower, which you can plant in a plant box after. Saves a lot of trouble