Here's how you can obtain a Moon Stone in Pokemon FireRed:
1. Find one in a hidden location: You can find a Moon Stone in the following locations:
* One Island: In the house to the north of the Pokemon Center.
* Two Island: On the east side of the island, by the waterfall.
* Four Island: You can find one on the island's northern side.
2. Trade with another player: You can trade with another player who has a Moon Stone.
3. Use the Game Corner: You can earn enough coins at the Game Corner to exchange them for a Moon Stone.
Once you have a Moon Stone, you can use it to evolve certain Pokemon, such as:
* Nidorina -> Nidoqueen
* Nidorino -> Nidoking
* Clefairy -> Clefable
* Jigglypuff -> Wigglytuff
Remember that Moon Stones can only be used on certain Pokemon. Make sure to check which Pokemon can evolve with a Moon Stone before attempting to evolve them.