How do you get mist stone in Pokemon FireRed?

You can't get Mist Stone in Pokemon FireRed.

Mist Stone is a held item introduced in Pokemon Diamond and Pearl. It is not available in Pokemon FireRed.

If you are looking for a similar item in FireRed, you might be thinking of the Everstone, which prevents Pokemon from evolving. It can be found in the Celadon Department Store, on the 4th floor, inside a drawer.