Best answer: Wraiths Kunai and her other heirlooms are only available through Apex Packs. There is, we are told, a one-in-500 chance of you receiving it so all you can really do is open as many Apex Packs as possible. You can win them or buy them but you will need a lot of them.
What is Wraiths Kunai?
Wraiths Kunai is part of the heirloom set available in Apex Legends. It features three unique items — Wraiths Kunai, a banner pose showing the Kunai, and an intro quip soundbyte — that can be equipped to wraith to customize her look and sound. Each legend will eventually have its own set of heirlooms, but for now, only Wraith has them.
It is important to note that the Kunai does not change your attacks in any way. It is purely a cosmetic item, replacing Wraith’s normal melee attack with one that looks way cooler.