Boo Finder

The Boo Finder is an item in Luigi's Mansion 3 that can be purchased from Professor Elvin Gadd's Shopping Network for 1000G. Once bought, it will reveal the location of one Boo on the player’s map.

Luigi's Mansion 3 description
E. Gadd's Shopping Network: "That's a cartridge that'll project the location of a Boo on to your map. Each one reveals the location of one Boo!"