According to Bulbapedia, a scientist in the house directly above the Pokemon Center in Geosenge Town will give you an Everstone the first time you talk to her.
Other repeatable methods include:
Battling Psychic Inver on Route 18
Held by wild Geodude, wild Graveler (5% chance each), wild Roggenrola, and wild Boldore (50% chance each)
Other than the lone Everstone in Geosenge Town, your best bet to stack up on Everstones is to catch or use Thief, Trick, or Switcheroo on wild Roggenrola in Reflection Cave.
Hope I could help!