There is a chance that boxed Pokémon in Poké Pelago (isle Aphun) can find Dawn Stones.
Other than that, Hau'oli city in the post-game is the only place where you can find one. You need to fight Guzma near the Hau'oli beach after becoming the champion. If Guzma isn't there, you need to find him in his house on Route 2 and talk to him there first. After the battle (his mons are around level 63 btw), he will give you the Dawn Stone.
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