There are 5 guaranteed Moon Stones you can obtain in Pokémon FireRed in a normal playthrough.

Here’s where they are:

  • 2 in Mt. Moon (one visible item, one hidden).
  • 1 in the Team Rocket Hideout under the Celadon Game Corner.
  • 1 in the Pokémon Mansion on Cinnabar Island.
  • 1 rewarded on Two Island after completing Lostelle’s side quest (her father gives it to you).

In addition, wild Clefairy on Mt. Moon have a small chance to hold a Moon Stone, so you can potentially get extra ones by catching them or using Thief, but those are not guaranteed the way the five fixed locations are.