You can get pvp gear from any of the Celestial bosses on timeless isle. You don't even have to be 90. Just run up and punch one of them in the face when a raid is attacking and you might get tier 16 or pvp gear.
The four Celestial bosses share a single weekly loot drop. You can get loot from the first one you defeat per week (and another if you use a bonus roll), but after the first defeat none of them will drop anything for that character again until the next raid reset.