During Chapter 5 of the Main Story, you will unlock Clan Battles. Once this has been unlocked, continue of with the story and your missions until after you must fight and defeat Toru. Once Toru has been taken care, a scene will begin that will show La Pente's Bartender mentioning to Kiryu of a contact of his that is in the business of selling. This character is known as the Black Market Merchant, and he actually sells some of the rarest and bets items found in the entire game, the same that you can only buy exclusively from him.
In order to find the Black Market Merchant, you will need to proceed with the game as usual after the bartender tells you of him. Eventually, the bartender will send you a SNS on your Phone, mentioning how the Black Merchant said it was okay for you to visit him. Once this has happened, you will be able to find the Black Market Merchant near the Safe in Kana-no-kubo.
Kana-no-kubo Black Market Merchant Items:
Custom Mouth Guard
Dragon's Binding
Military-Grade Body Armor
Beads of Exorcism
Parry Talisman
Ahab's Revenge
Tauriner Maximum
Tauriner+++
Staminan Spark
Staminan Royale
AppStimRX
Later in the the game, the Kana-no-kubo Black Market Merchant will actually set you up with being able to find his friend, the Kamurucho Black Market Merchant. After this has been done, you can find this Black Market Merchant in the sewers of Kamurucho. Unlike the first Black Market Merchant, this one will sell you a number of different items.
Kamurucho Black Market Merchant Items:
Transcendent Mouth Guard
Military-Grade Bulletproof Vest
Transcendent Body Armor
Chain Mail
Transcendent Insoles
Dragon God Amulet
Beads of Exorcism
Tauriner Maximum
Staminan Spark
AppStimRX
Tauriner+++
Staminan Royale