There is one in Nimbasa City and Relic Castle.
If you had Black Version, another option would be Black City, so if you have a friend who has Black Version and can get to Black City, you could ask them to trade/get you one. This can be done by attaching it to a Pokemon (that doesn't evolve via dawn stone) and trading it over.
If you don't want to permanently send a Pokemon over, after you receive the Pokemon holding the Dawn Stone, exit the trade, remove the dawn stone, then re-trade the Pokemon.
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on route 225 there will be a dawn stone
Get one eevee and it will be held the dawn stone
Get a male Kirlia, NOT Gardevoire, and use a dawn stone on it. First off, make sure you have a male version of kirlia. Then use a dawn stone on kirlia. Kirlia should then evolve into Gallade.
You can purchase a Dawn Stone at the Pokeathlon with points on certain days.
You cannot get the Dawn Stone in Pokemon Emerald, as it is first introduced in Pokemon Diamond and Pearl