How you do this:
Okay when you first got to that patch of land BEFORE entering Canalave City what did you see? Right! I medium field of grass! Once you have the Poke Radar go there and activate it. Go to the grass patch that shook closest to you. |NOTE: TO GET THE POKE RADAR YOU NEED THE NATIONAL DEX|| It might take a few tries but once you find it, make sure you have a pokemon who has a status changing move FIRST in your party and then when you use sleep powder, or whatever you used, simply catch it with an ultra ball just to be safe. You MIGHT fail but keep trying, it doesn't flee on you so you shouldn't worry.