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! ;)

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)