in celadon city, go into the game corner and push the switch on the poster to the right of the desk and stairs will pop up and that is the entrance to the team rocket warehouse
To get into the Team Rocket Warehouse, go to the game corner and beat the Team Rocket Grunt that is standing by the poster. Beat him in battle then go down the steps after you turn the switch behind...
You have to get to the far right , fight everyone and get the sapphire. If you don't have the codes, then save lorelei, use cut on the dotted hole and get it where the scientist will steal it.