DIAMOND/PEARL/PLATINUM spots:
I like Valley Windworks - Bidoof (1 EV) and Shellos (1 EV) are common here. Gastrodon (2 EVs) can be seen surfing above here. Bidoof are most places, like Route 201.
Wooper (1 EV) and Quagsire (2 EVs) are also on Route 212, higher level.
HEARTGOLD/SOULSILVER spots:
Surf inside Slowpoke Well to find Slowpoke (duh!) (1 EV)
There's also Route 15, which has Nidorina (2 EVs), Noctowl (2 EVs), Quagsire (2 EVs) and Chansey (2 EVs).