It is found at the Celadon Game Corner and must be purchased for 4000 coins.
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TM35
It learns Flamethrower at level 34.
TM 35 Flamethrower can be brought from the Celadon Game Corner.
To get TM35 - flamethrower in FireRed, you have to go to Celadon City and win it for prize coins at the casino. I think it costs 4000 prize coins, so you'll have to play at the casino a lot or waste a lot of money buying coins.
You can "purchase" a flamethrower TM in the game corner coin-tradey place.