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.

When can you catch Dreepy?

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. Yeah, those aren't brilliant odds. Thankfully, you can also encounter Drakloak, its first evolution in the wild.

How do I get Dreepy early?

While Dreepy and Drakloak can be found on the Lake of Outrage, which can't be accessed until the water attachment is obtained for the Rotom bike, Dreepy can be found much earlier in a Raid Den.