where to turn in tier 16 tokens?
I don't know but the vendors near the Temple of Niuzao might sell the new tier 16. They sold tier 14&15 so far so could be a possible answer.
Signature by Yonie | 5.4 Elemental Shaman DPS Guide - Warcraft Academy
Shrine, where they exhibit the challenge mode sets. [at least that's where they were for alliance on the PTR]