it's in the gym where u vs. misty
Power Plant
you have to have completed the power plant mission and he will be outside the power plant on the left
See below
The grass in Kanto by the Power Plant.
it is i the kanto region in soul silver after you defeat the elite 4
I don't know when but after a while the power plant
Battle Misty
At the Power Plant.
start by heading north and taking route 10 to the power plant
It's in mistys gym
you go to the cerulean gym by the floaties on the left after take it back to power plant
the power plant solution or go and awake the snorlax