If you need these or if you need quick cash, try camping the Water elemental spawn that's near the OP in the marshlands; it's almost always rainy there.
Another good place to get these is the Labyrinth of Onzozo. Rainy weather is common there and there are about 4 or 5 water elementals in the area around where Ose spawns.
I recommend the clusters up in Ru'Aun Gardens, granted be ready to fight for your life when fighting them cause they semi-spam Agas and Ancient Magic associated with its own element
