An Everstone can be found in Geosenge Town from the Scientist in the house northwest of the Pokémon Center.

If you would like to farm for more, Psychic Inver can be battled in the Inverse House on Route 18. When beating Inver in an Inverse Battle, you have a chance of receiving an Everstone from him upon reaching a score of 7-9. This score is calculated from the number of Not Very Effective moves used subtracted by the number of Super-Effective moves used.
Additionally, the Everstone can be found held by Geodude and Graveler at a 5% chance and Roggenrola and Boldore (which can only be found in the Friend Safari) at a 50% chance.
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