Pokémon Mystery Dungeon: Blue Rescue Team and Red Rescue Team/Darknight Relic

This 15 floor dungeon is unlocked by purchasing the Secretive Forest Friend Area.

All of the Pokemon here are at least part Ghost type, so most of them can travel and attack through walls. Also, having the Super Mobile IQ skill(or a Ghost type of your own, preferably with a Tight Band) will allow you to find items in the walls of some of the floors.

Items
Rare TMs and Wonder Orbs can be found in the walls of floors 3, 5, 7, 9, 12, and 15. Other than that, mostly just standard issue dungeon items(money, common orbs, stuff like that).