DIAMOND/PEARL/PLATINUM spots:
Try Starly (1 EV) on Route 201.
Or there are plenty of pokemon in the Trophy Garden: Pikachu (2 EVs), Pichu (1 EV) and Staravia (2 EVs) are fairly common, and some of the daily pokemon give off Speed EVs too.
HEARTGOLD/SOULSILVER spots:
Route 9 is great! You get Rattata (1 EV), Raticate (2 EVs), Spearow (1 EV) and Fearow (2 EVs).