The only place that you can find Dreepy is in the Wild Area, more specifically the Lake of Outrage subsection in the upper part by Hammerlocke City. Crossing the body of water you will see a tiny patch of grass that wild Pokemon will spawn in.

What weather does Dreepy spawn in?

Dreepy has a 1% chance of spawning as a non-overworld encounter (so as an exclamation point in the tall grass) in Overcast weather, and a 2% chance in Heavy Fog and a Thunderstorm.

Where do you find Dreepy?

Dreepy Location can only be located in the Wild Area. Specifically the Lake of Outrage section in the top left corner. South of Hammerlocke. It has a two percent chance of appearing in Wild Grass patches.