Per the wiki (http://wiki.guildwars2.com/wiki/Legendary_precursor), only Mystic Forge gambling, Claw of Jormag chest, Orr Temple chests, and dungeon chests can drop precursors.
It’s been shown anecdotally that there is a very slim chance of a random world drop producing one as well.
The reason I bring this up, is that last night, we were debating whether or not the Shatterer chest/Tequatl chest will produce one or not. Can anyone provide an authoritative answer either way on that?