buy it at the celedon city shop
You can buy one at the department Store in Celadon City.
evee can evole into three things in firered and leafgreen. it can evolve into flareon if you use the fire stone, jolteon if you use the thunder stone, or vaporeon if you use the water stone.
Either try a thunder stone or just train it like there's no tomarrow.
the Pokemon that evolve with the help of a thunder stone are : pikachu +thunder stone = raichu, or eevee + thunder stone = jolteon.
The TM that contains Thunder is placed in the Power Plant.
thunder Pokemon if it has evolutions
The TM thunder is found in the power plant.
You can find the thunder stone in New Mauville
you can find a thunder stone by checking the UNDERGROUND
Thunder Stones can be purchased at the Celadon City Dept Store on the 4th Floor.
That TM is hidden in the power plant.
You first need a yellow shard these can be found using Dive near the Hunter's house, Second give the yellow shard to the hunter he will trade you a thunder stone for the shard. Another way to get a thunder stone is by trading a Pokemon from firered or leafgreen that is holding one to ruby, sapphire or emerald.