Patch 5.4.0 (2013-09-10): All bosses in Throne of Thunder now have a chance to drop either a Sigil of Power or Sigil of Wisdom. The first eight bosses in Siege of Orgrimmar have a chance to drop a Sigil of Power, Sigil of Wisdom, Secrets of the Empire, or Titan Runestone.
Patch 5.3.0 (2013-05-21): The Strength of One's Foes: All bosses in Mogu'shan Vaults, Heart of Fear, and Terrace of the Endless Spring now have a chance to drop either a Sigil of Power or Sigil of Wisdom.
Hotfix (2013-01-22): "Sigils of Power now only drop in Mogu'shan Vaults, and Sigils of Wisdom will only drop in Heart of Fear and Terrace of Endless Spring, with a very small chance to drop in LFR Mogu'shan Vaults. This should ensure that players who had access to Mogu'shan Vaults for a longer period of time are now seeing more Sigils of Wisdom."