The earliest place to get Dawn Stones is Via Super Training. It’s not the easiest, but it’s by far the earliest, as you can get it before the first gym.
You can do super trainings before the first gym, but it’s entirely luck based and will take a while to obtain. You have to have Max EV’s on a Pokémon and complete all basic courses to unlock “Those Long Shots are no Long Shot!”, which you have a chance to win a Dawn Stone.
You can also get one in Victory Road, Sea Mauville, and from a Secret Pal in a Secret Base. The first two would be the easiest, but they’re far into the game.
The easiest way though, would probably be by hacking or trading for one. Since those are likely not an option for you, you might want to get your Super Training Gear out, or just suffer through the end of the game.
Source: Serebii and Experience (did an oak challenge; got Gallade before the first gym)
Also this