Fescue City

South of Plantain Path and North of Fatsia Path

After entering the city, speak to a mysterious businessman who will give you $1000 after some dialogue

An old man on the second floor of the building in the upper right-hand corner gives the player a Chesto Berry

There is also a PokeCenter that has a mart built in. The man on the right sells Ultraballs, but not Pokeballs or Greatballs for some reason.

After speaking with the businessman and getting your money, return back to the lab and speak with your lab assistant Tyra.