These drop in piles of 3-5 (at least in my experience) from elementals at the mage-exclusive abyss/sky locations. You can trade 20 of them for a 1g coin at the Architect's shop, so each one is basically an extra 5s. Free money, woo! You can also obtain them by killing enemy players at the abyss/sky (outside of battles for control of the location) when they have them, but since this almost never happens in practice I don't know if it transfers all of them or only some of them.