he doesn't actually exist
Nowhere.
You can trade an Abra for Mr. Mime in a house on Route 3 in Pokemon FireRed.
rescuing Mr. Fuji from the Pokemon Tower.
Go to Mr Psychic's house
Use the pokeflute that Mr Fuji gave you.
Use the itemfinder where Mr. Fuji was in the Pokemon Tower.
rescue Mr. Fuji and he will give you the pokeflute - by hezekiah
mr. fuji gives you the poke flute when you rescue him from team rocket at the Pokemon tower
This TM is given by Mr psychic in saffron city his house is to the right from the Pokemon center.
using the intem finder while standing where Mr. Fuji was in the Pokemon tower
find him by the gate
You can get it from Mr. Psychic. He lives in one of the houses to the south in Saffron City.