To get this, you must purchase three specific items from the Raid Mode Store. In the Raid mode main menu, select
Store and scroll through the list until you find them.
Rare Finder 1 - 120,000 BP
Rare Finder 2 - 150,000 BP
Rare Finder 3 - 200,000 BP
Item description:
Ups your chance of finding rare weapons in weapon cases slightly.
The total cost of all three items is
470,000 BP. You'll have to complete numerous Raid Mode stages to earn that much. Be sure to shoot the medallions that occasionally appear throughout the levels, as they provide a BP bonus. You also earn more BP for S Ranks, so try to have good accuracy and avoid taking damage (of course).