You can give your pokemon a specific power bracer to train for particular ev...
Also I would recommend you to have a pokemon infected with pokerus which will double the rate of gaining evs...
My places are:
attack:Peridot alleyways (pochyena and mightyena)
Sp attack:Pyroas mountain (slugma and numel)
Hp:Peridot alleyways (all the mons other than pochyena and mightyena)
Def:Apophyll cave (geodude and graveler)
Sp def: azurine lake (tentacool and tentacruel)
Speed: Swoobat,woobat and noibat