Well, I wouldn't say there is Really say there is a best place to find anyone shiny, really, Shiny Pokemon are those of a different colour, with it rarity, it's really just a way of testing your patience, but to help, if you didn't already know where Haunter is available, here is where you can find it: Frost Cavern, Victory Road, 14 (Laverre Nature Trail), 19 (Grand Vallée Way). I really doubt it would be easier to find a shiny for one specific area, but as long as you have undying patience, you will find the Shiny Haunter you desire, just remember to have a good supply of Pokeballs on you, I hate for you to encounter one and run out of or not have any Pokeballs to begin with. Good Luck
In case you forgot what a shinny Haunter Looks like >>> http://pokemondb.net/pokedex/haunter <<<