15k Armor typically refers to Elite armor. Occasionally some people use the term to mean both elite armor and Obsidian armor, as they have identical gold costs. Obsidian armor is also referred to as FoW or Fissure Armor, since it is only obtainable from the Fissure of Woe. You can also find 15k armor in The Granite Citadel, Marhan's Grotto, Vasburg Armory, Leviathan Pits, Divine Path, Command Post, and Throne of Secrets.