Drowzee can be caught on Route 34 (between Goldenrod and Ilex Forest) at level 10-12, and learns Headbutt naturally at level 15. Seel also learns it naturally at level 1, and can be found on the Whirl Islands (which requires Whirlpool, which you should be able to use as of the 7th Gym Badge).
Alternatively:
There’s a move tutor in Ilex Forest that will teach (or reteach) Pokémon Headbutt. If you’re after a throwaway Headbutt slave in the event that you’d rather not delete the move that deleted Headbutt to begin with, Poliwag can be fished from within the Forest (using any of the three Rods), and Drowzee can be taught Headbutt prior to 15.