Just to add to some of the EV data here. There are a few people to hit up with VS Seeker.

HP - Cowgirl Shelly - right below the Daycare center but before the bike ramp. She has 5 Bidoofs for 1 HP EV each.
Att - Sunyshore City, bike west to the 5 fisherman. The one on the far left has 2 Gyarados with 2 Att EVs each.
Def - Cave left of Oreburgh City - low level Geodudes with 1 Def EV each.
SpAtt - Scientist Shaun - Route 212 South West of Pastoria City. 2 Alakazam for 3 SpAtt EVs each.
SpDef - Ninja Boy Davido - Defog on Routh 210 east of Celestic Town. 1 Dustox for 3 SpDef EVs.
Speed - Fisherman Andrew - Routh 205 west of Eterna City (2nd fisherman on the dock) - 6 Magikarp for 1 Speed each. (said earlier by Nexus)

Changing power items when training at these sites also helps to speed up your EV training. ie- your training for Att and Speed, you can train at Fisherman Anderw with a Power Bracer until you hit your EVs needed for Attack and then finish off what you need for speed (subtracting the 1 or 2 EVs your receiving while battling for Attack EVs). Just make sure you calculate everything in advanced and know how many Magikarps you need to face.

well the only useful ones are cowgirl Shelly and fisherman Andrew. BUT Shelly isn't in platinum version (so I've been told)

and some people are too impatient to wait for Andrew to want to fight again with VS seeker


That's why I didn't include either.

Training Attack EVs

Head
East of Celestic town. There you'll find a patch of grass which you'll
encounter Machop for 1 point, Machoke for 2 points, and Bibarel for 2
points. There is also a chance to find meditite but it gives 1 speed
point.

Training Defense EVs

The best
place to do this is in the Ruin Maniacs cave. This is outside of where
you catch the Unknown, the tunnel you actually find the guy in. Here
you will find primarily Geodudes, though Hippotas are not completely
uncommon. Each of these guys give 1 EV point each for defense.

Training Special Attack

The
best place to train this is in the Resort Area. In this small little
town you will find a small pool of water near the bottom right-hand
corner of the town. In this pool of water you may surf, and find
Golducks. These guys give a generous 2 EV points for everyone you
faint. On top of that, they are a 100% encounter rate.

Another
place you may train special attack is in the Old Chateau, where you
will encounter Gastlies who give 1 point each 100% encounter rate. If
you have a GBA game inserted into your DS, you will also encounter
Haunter and Genagr. These guys give 2 and 3 special attack EVs
respectively.

Training Special Defense

The
best place to train this stat, is North of Sunyshore city. Surf in the
water and you'll find Tentacruels who give 2 ev points in special
defence. They are a 60% encounter rate. You will occasionally run into
Mantykes, who gives 1 Special Defense EV each.

An alternative
and possibly better place to train is in Pastoria City. East of the
Move Tutors's home you will see a pool of water. If you surf in here
you will encounter Tentacools which give 1 special defense EV each.
This is a 100% encounter.

Training Speed

The
best way is using the Versus Seeker. On Route 205, you will find many
fishermen fishing. (You get to Route 205 by exiting Eterna City via the
eastern exit.) Challenege the second fisherman from the right,
Fisherman Andrew. He has 6 Magikarps that give 1 Speed EV each.

hey is it the same for platinum???
hacing trouble finding a good spot to ev train in speed