Pichu #1 - Hidden outside Booker's House. Find by using Roar
Pichu #2 - Meets you when you cross the bridge leading to the Wireless Tower
Pichu #3 - In a tree in Teakwood Forest just outside the exit of Rasp Cavern. Needs Slam * 1
Pichu #4 - Clinging to a cliffface at the end of Dangerous Cliff
The next two require you to go to the Aqua Resort where you'll see them flying off while riding Staravia
Pichu #5 - Falls from the sky in Silver Falls
Pichu #6 - Fly from Hauldera Volcano and you'll encounter it and automatically land.