Where to find it? Edit
Once you have completed Shelley's gym you will want to head underneath the Grand Stairway and grab TMX Rock Smash, from the cave. Once you have done that you can go the Abandoned Yureyu Power Plant to the far right of the Beryl Bridge. Below the Entrance, to the power plant, there will be a metal gate with a small opening guarded by a rock. Use rock smash on it and you will enter Ametrine Mountain. NOTE: This is a different part of Ametrine than you will encounter later in the game. Once inside you can find and catch Alolan Sandshrew by simply walking around. It has a 9% chance of appearing within the mountain.