There are 5 ways to get a Dawn Stone and most of them are repeatable. The previous comment listed every possibly acquisition method properly. But just to make it simpler:
The easiest way to get a stone is to get the one you get from Wally in Victory Road. This is obtained just by playing through the game.
But the fastest way is to do the Secret Super Training “Those Long Shots Are No Long Shot!” stage. The prize pool is 5 items where the Dawn Stone is one of them. You gain access to Super Secret training immediately after Wally captures his Ralts, so it's available very early in ORAS. But you'll need a Pokémon with fully-trained EVs before you can do this.
Alternatively, the 2nd fastest way might be via the Inverse Battle Shop in Mauville City.
Source: https://guidestrats.com/pokemon-oras-dawn-stones/
