The easiest way to get stones in Alpha Sapphire (I assume that's what you meant since dusk stone doesn't exist in Sapphire version) is super secret training level 6 "Watch Out! That's One Tricky Second Half"
In your botom screen select the secret training app. Get a Pokemons EV's fully trained. Then you will unlock super secret training. Beat all the levels in super secret training up to number 6. When you get to the one titled "Watch Out! That's One Tricky Second Half" keep playing it until you get the dusk stone as a reward. No tm's hm's or minimum level required.
Check out dusk stone [here] for more options on obtaining.
And as @V-Man pointed out you want a dawn stone to evolve Kirlia not a dusk stone. Same directions except you want level 7 "Those Long Shots Are No Long Shot!" not level 6.
If you really did mean "Sapphire" not Alpha Sapphire you are out of luck. Dawn and Dusk Stones and Gallade were all introduced in Generation 4 and Sapphire is Generation 3. So if you really did mean "Sapphire" your out of luck. There is no Gallade.