I saw that everyone think that Premier Balls cannot be found anymore but this isn't true at all ! You can still found one Premier Ball for each character your create and pretty fast !
Steps :
1 - Go to Rustboro City.
2 - Go in the house just below the Mart
3 - Take the stairs to the second floor.
4 - Talk to the kid.
5 - Enjoy your free Premier Ball !
Edit : @Soul also sait it here before me but I didn't saw it in time.