Key to the Palace of Lei Shen

A Key to the Palace of Lei Shen can be used to start a Troves of the Thunder King treasure run by speaking with Taoshi in the Bleak Hollow on the Isle of Thunder.

Source
This item has a very low chance to be found inside Arcane Trove or Sunreaver Bounty. Aditionally, rogues can loot these keys from Sparkling Sets of Keys in the Hall of Shadows.

The "other select sources" would be:
 * Trove of the Thunder King treasure chest anywhere on the Isle of Thunder (~25%)
 * Arcane Trove or Sunreaver Bounty (~3%)

Quests

 * Treasures of the Thunder King (quest)