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In the following locations, there is a high (sometimes even a 100%) chance of finding only the Pokemon listed.

Attack: Lake Verity (located northwest of Twinleaf Town)

When you obtain the Super Rod, fish in the water in Lake Verity. The only Pokemon you will catch are Seaking and Gyarados, which both give out 2 EVs in Attack. A Pokemon around Level 50 is recommended. To get the Super Rod, talk to the fisherman in the Fight Area. The Fight Area is only accessible after you beat the Elite 4.

Defense: Oreburgh Mine (located south of Oreburgh City)

In Oreburgh Mine, you will mostly find Geodude, which give out 1 EV in Defense. Sometimes an Onix will appear, which also gives out 1 EV in Defense. The only Pokemon to avoid is Zubat, which gives out 1 EV in Speed.

Special Attack: Old Chateau (located near the eastern exit of Eterna Forest)

You will only find Gastly, Haunter, and Gengar in this chateau (not inculding the Rotom from the TV). They give out 1, 2, and 3 EVs in Special Attack, respectively. You can find Haunter in any room as long as you have a Game Boy Advance game in the second slot of the DS. You will only find Gengar in the room with the painting with the red eyes if you have a Game Boy Advance Pokemon game in the second slot of the DS. For both Haunter and Gengar to appear, you also have to defeat the Elite 4.
(Credit goes to FullmetalSasuk3)

Speed: Victory Road

In Victory Road, in the room when you're first able to surf, surf on the water. The only Pokemon that will appear are Golbats, which give out 2 EVs in Speed each.

If anyone knows good places to EV train for HP or Special Defense that has practically these odds of finding specific Pokemon to train against, please list them here. If you are a crew member and you want to add something, you may edit this post.