Walkthrough
Directly after making it through Victory Road, you will come to Indigo Plateau. This is the location where you can challenge the Elite Four in order to become the next Kanto Region Champion. Make sure to stop by the Pokemon Center and Poke Mart located within Indigo Plateau before moving ahead to facing the Elite Four, as you must face the Elite Four consecutively and won't have the chance to heal your Pokemon except by using your own items.
Poke Mart Items
Ultra Ball, Great Ball, Full Restore, Max Potion, Revive, Full Heal, Max Repel