So there are 4 different ways to get Everstones in the game:
1. Granite Cave (B2F) - A hidden item on the big rock that's directly to the left of the hiker NPC. The source link below has a step-by-step tutorial showing you exactly how to reach this item.
2. Super-Secret Base - “Pick up Stones” skill with a Secret Pal
3. Inverse Battle Stop Reward for earning between 7-9 points in Mauville City
4. Held item on wild Geodude & Graveler (5% chance), or on wild Roggenrola or Boldore (50% chance)
The easiest one to get is probably the one in Granite Cave. After that, how you'd get more would mostly depend on what you'd want to grind for them.
Source: https://guidestrats.com/pokemon-oras-everstones/