I would like to know this too:)
There is no TM Earth Power. Therefore you cannot get it.
If you mean TM 26 Earthquake, then you can find it in Victory Road or get it for 80 BP.
Good luck and don't forget to BA! :D
This is something you can find by Googling HGSS TMs list.
Serebii - HGSS TMs
Earth Power doesn't exist as a TM in any generation, only existing via level up, breeding or taught by move tutor. You don't get the move tutor until you've beaten the game.