The Mana Seed is an important part of The Mana World's history. It is also the basis of the first quest required in order to begin using Magic.
The Forest Mana Seed can be found in a cave NorthWest of Hurnscald. To find it, head West from the Soul Menhir of Hurnscald. At the first fork (at a farm with many trees), go North. Continue North until the path forks again. Take the West fork and follow the path to a set of caves, one higher and one lower. Enter the higher cave, and continue through until it exits. Continue East along the stones in front of the cascade and into a second cave. The Forest Mana Seed will be in the NorthEast corner.
There is also a Desert Mana Seed located at the Noble Quarters in Tulimshar. It doesn't have any use yet. Spell taught by the Mana Seed:
You can find the script here.