Where can I EV train my Pokemon for Special Defense?

Question

What are the best locations for getting Effort Values in Special Defense?

Answer 1

DIAMOND/PEARL/PLATINUM spots:
Surfing in most places you encounter Tentacool (1 EV) and Tentacruel (2 EVs) who give off Sp.Def EVs!

HEARTGOLD/SOULSILVER spots:
Surfing, same as above.

Comments

Nice job answering yourself! ;)

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If you surf at Route 223, then you'll encounter Mantyke as well as Tentacruel.

Answer 2

Black / White:

ROUTE 17:
Frillish 1 EV (surf)

Answer 3

Pokemon Heartgold/Soulsilver:

Surf along certain routes and you will find nothing but Tentacool and Tentacruel in the water:

Route 26,
Route 27,
Route 34,
Route 40,
Route 41(plus Mantine in Heartgold only- 2 SpDef EV's)

Also, you can find them in cities or other significant locations by surfing, too:

New Bark Town (off the shore on the right);
Cherrygrove City (off the shore on the left);
Olivine City (off the harbor to the south);
Cianwood City (off the shore farthest north);
Cinnabar Island (around the island);
Pallet Town (off the start of the inlet to the fence)

Answer 4

Black/White 2 - Special Defense spots

Surfing on Route 17 - Frillish (1), Jellicent (2) in bubbling spots.

Or possibly Route 14 - Golduck (2), Swablu (1), Altaria (2). Avoid the rest.

Comments

Golduck gives 2 Special Attack Effort Values.

Answer 5

In Pokemon Black and White I've found that the volcarona room only spawns claydols which give 2 Special Defense EVs.

Scource: Bulbapedia and Experience

Answer 6

RSE locations

Walking at Mt. Pyre: Duskull (1 EV) (doesn't work in Sapphire or Emerald)
Super rod at Abandoned Ship: Tentacool (1 EV), Tentacruel (2 EVs)