Ilex Forest and Viridian Forest.
For more information go to this site great pokedex.
http://pokemondb.net/pokedex/spoink
Also, you can just check http://pokemondb.net/pokedex for locations etc for the desired Pokémon, you don't have to ask this.
on a wednesday go to either Ilex Forest or Viridian Forest, go to your pokegear and hit the top left button in the radio and Hoenn music should start to play, now encounter a pokemon and if it's not spoink just put on the music again and encounter another, do this until you find a Spoink.
