Blaze Spirit

I finally got mine from the elite Fire Elementals in Vakthros Range, Azure Span (around 77,39). It probably took around 30 tries. There are a lot of these elementals present, but aim for the edge ones and avoid patrolling mobs. Be aware that they occasionally cast an armour buff (which can mess up your capture if it uses it in its last few % health) and also an explosion that can stun and interrupt you. I found my BM hunter to be best, as he could remove the armour buff, stay at range to avoid the interrupt/stun, and use pet DPS to kill the elemental while channeling the Zapthrottle Soul Inhaler through its last few % health. Being a hunter also allows you to feign death if things go badly, and they will now and then as the spells it uses seem randomly timed.

So my mileage varied.  I was 1 drop for 2 Frosty Souls for the Tide Spirit, 1 drop for 66 Airy Souls for the Gale Spirit, 1 drop for 80 Fiery Souls for the Blaze Spirit, and lastly, 1 drop for 99 Earthen Souls for the Dust Spirit.  Although obviously doable, this is a disagreeable grind, if done solo, you have to end your kill with a DoT spell and then time the dying soul capture to avoid the knock backs and stuns all these rares possess.  Easier said than done.  Good luck.