Just takes some time lol, i found mine in like 3 minutes.. Before the game was rolled back and when the Christmas event was running it took me 2 hours to get a normal pichu
You can't find Pichu's in the night maps, only Pichu (Notched) can be found in the night electric and grass maps. To find Pichu's you need for the event you need to search in the day electric and grass maps.