There are two places to do this. The first is in the Game Corner Basement. Go to the guy with the lift key (he's the on one that says "The elevator doesn't work? Who has the LIFT KEY?") and fight him. Afterwards, get on his left side and talk to him again. You are now in a poké ball. To get out simply move over in any direction. The second is also in the Game Corner Basement. Just talk to Giovanni from behind and, after the battle, you will be in a Poké ball.