You buy it from Licia of Lindeldt in Heide's Tower of Flame, right after the Dragonrider boss fight.
You can also find it in a chest (Mimic) in Earthen Peak, behind a hidden wall.
"From the room where Mild Mannered Pate is, you can exit through the door that you opened when grabbing Soul arrow and go right. Drop down one level and you will find a chest in a hidden area in the wall with Lightning Spear."