I will continue to update this as I find better places but here's a first draft. The numbers in brackets are the number of EVs you get from that Pokemon.
HP spots
Surfing in Icirrus City - Stunfisk (2) everywhere! Seismitoad (3) can also appear in bubbling spots.
ATTACK spots
Dragonspiral Tower 1F - Mienshao (2), Druddigon (2), Golurk (2).
Victory Road 1F/3F - Banette (2), Golurk (2)
N's Castle - Druddigon (2), Zweilous (2), Excadrill (2) in dust clouds. Avoid Onix (dust clouds).
DEFENSE spots
Chargestone Cave - Nosepass (1), Klink (1), Ferroseed (1). Avoid the rest (note: Boldore gives 1 Def and 1 Atk EV).
SPECIAL ATTACK spots
Celestial Tower - Litwick (1), Elgyem (1). Stick to the first floor to avoid Golbats.
SPECIAL DEFENSE spots
Surfing on Route 17 - Frillish (1), Jellicent (2) in bubbling spots.
Or possibly Route 14 - Swablu (1), Altaria (2). But a mix of a few other Pokemon to avoid.
SPEED spots
Surfing in most places (e.g. Route 1, Wellspring Cave) yields all Basculin (2). Feebas (1) is in bubbling spots but avoid Milotic (2 Sp.Def EVs).