Well, precious metals (gold and silver) and the few treasure statues were in the game for a long time. So you should know where to get these.
The mini treasures randomly drop from enemies.
Heroic treasures are found in big npc camps (Black Galleon, New Asgard, etc) Each of these has multiple spots where a treasure can spawn. I am pretty sure only one can spawn at a time, but can’t confirm atm.