Ancient Mementos (aka "mems" or "mentos") are dropped by most dungeon bosses in higher-level dungeons. The first dungeon in the game's progression that drops them is Heart of the Ocean, a level 55 6-player dungeon.
Every player in the group or raid is able to loot the same Ancient Mementos from a boss.
Ultimately, this information needs to be on the Dungeon/Zone page only. This page should just contain a list of dungeons that reward mems.
Bosses in the following dungeons drop Ancient Mementos (See each page, top of the Bosses section, for a table of Bosses by difficulty, and how many mems they drop.):