It Was Worth Every Ritual Stone

Spirit Champions
Echo of Kros located at 55.2 87.7 - island stage 1
Spirit of Warlord Teng located at 35.7 63.8 - island stage 1
Kor'dok located at 30.7 58.6 - island stage 1

Lightning Champions
Electromancer Ju'le located at 44.5 61.0 - island stage 1
Qj'nor located at 48 26
Ancient Mogu Guardian located at 55.3 47.9

Primal Champions
Cera located at 57.9 79.2 - island stage 1
Windweaver Akil'amon located at 68.9 39.3
Incomplete Drakkari Colossu located at 49.9 20.7

If you are exalted with your Isle Of Thunder faction and have a ton of those Lesser Charms, like about 5k of them, you can buy 20 Elder Charms (which is the cap) with 20 for each on Commander Lo Ping, one of the Quartermaster on Niu Zao Temple, Townlong Steppes.

Then head to your faction Quartermaster on the Isle of Thunder (Vasarin/Hiren) and buy a Sunreaver Bounty/Arcane Trove with 15 Elder Charms. You will mostly win about 5 Charms back and other stuff (got one on 3rd attempt).
According to wowhead, there's about 20 % chance you'll get a stone from either and each boxes, which seems more promising than stalking all rares on the Island, specially if there may be other people killing them too.

Also you can buy the boxes on Supplier Bao near Taran'zhu, but the Quartermasters are closer to the portals back to main land, which saves you some time too.

In order to summon a boss on the Isle of Thunder, you must first collect three [item=94221]'s. Although there are other sources—they're reliably obtainable, once per week, from the rare elites and treasure chests scattered across the island. For a map identifying the location of each bosses' respective ritual stone and a preview of what to expect upon encountering them, take a look at this guide.

Make sure to pickup Champions of the Thunder King prior to summoning; it can be completed weekly.

Caution:
These encounters are intended to be undertaken with the help of up to four additional players. Depending upon gear level, class, and specialization, it may be possible to solo—at least some of them. However, keep in mind that if you die, they'll despawn and you'll have lost your stones.

At the moment, none of these bosses are leashed to their spawn points and will therefore not reset with range. This may allow you to mitigate many of their mechanics through kiting.