I got one today from either a Gurite Piller or a Gurite Ambusher in the Northeast area of Lough Gur.
There's one of those stone things with the jewel on it straight north of the Parthenon Farm there. This is where Brannigan spawns for the Lucky Gold Coin one-time drop. The Gurites are on the hill right to the east of Brannigan's spot.