You have to go to kanto and got to the cinnabar and use your idem finder
Where In Pokemon Heart Gold Is The Electrizer Found?
Cerulean cave, if you don't know where that is, go on the Golden bridge at cerulean city and go to the little bit of land sticking out of the the cape filled with grass, surf down the water until you reach it. I advise bringing repels, a pokemon with flash and a pokemon with rock climb