You will get this with natural story progression.
However, there is an infinite Niskaru spawn available during "The Erathell's Blessing" side quest in Ayten if you want to knock this out quickly. Go to Ayten (first city along the road in the Plains of Etrathell) and talk to Singing Nell. She'll give you a quest to activate the five Windstones at Ayten Circle. The trick is to not activate them in the correct order, which is Lowest, Low, Medium, High, Highest. Starting at any one stone, activate each stone in a counter-clockwise motion (but not the correct pattern). This will summon a Niskaru. Defeat and repeat.