Destroyer cores
For cores, the best spot is Mount Maelstrom, though they are a rare drop. That being said a good reason why your drop rate is so low is probably because you have low magic find. You only really start seeing a difference when you have over 100%.
Destroyer Cores / Lodestones are random loot from chests in Sorrow’s Furnace, as well.
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Aren’t mobs in Mount Maelstrom too low level to drop? I heard onyx lodestones only drop from lvl75+ elementals, so I guessed that MM mobs would be too low for destroyer.