Received from a scientist in the same building as the person with a Patrat, if over 25 BW/40 B2W2 Pokémon have been seen in the Pokédex.
So you can get it if you see 40 Pokemon.
Available using Simulator γ
You can also get it in Dream Radar
Hope I helped! :) Dream Radar Bulbapedia Castelia City Bulbapedia