You get it from the Fuschia Gym leader
This TM doesn't exist in leafgreen.
There is no Tm trick in leaf green
you cant go underwater in leafgreen!!
Nowhere.
dig
Actually it's not a TM. You have to get a move tutor in FireRed or LeafGreen to teach it to a Pokemon.
Flamethrower is TM 35 and you can buy it in the celadon game corner
there is no zap cannon TM in no game and if someone told you that they are crazy.
chansey
In Victory Road.
Mt.moon
you get TM 26 earthquake when you beat the final jym in verdian city and he will give you the TM 26