you get to one island and go to the rocket brothers.
Give it to celio in the one island Pokemon center he will give you a rainbow pass which allows you to access islands 4-7.
it is on the mt ember
After Pokemon league you will go back to one island and go to the fire mountain and go right there is two Rockets beat them then go down in the cave and whola.
must have 70 Pokemon owned then go to the bottom of moutain ember go to the right and battle team roccet grunts