Organized by expansion & ease of obtaining:
Vanilla:
Questing in Swamp of Sorrows for Light's Embrace (A) or Dying Breath (H)
Questing in Searing Gorge for Lunk's Special Gear (takes a lot longer to get to this point)
Crafting Major/Minor Recombobulator w/ Engineering
BoE world drop Ankh of Life
BC:
Farming Heroic Magister's Terrace for Vial of the Sunwell
Wrath:
Malygos 25 for Living Ice Crystals
Blood Queen Lana'thel 25 for Bauble of True Blood
Cata:
Firelands trash drop, Scales of Life