Which high priest found the Book of the Law in the temple during Josiah's reign?

Answer: Hilkiah

Hilkiah the high priest found the Book of the Law and gave it to Shaphan the scribe, who read it.

“Then Hilkiah the high priest said to Shaphan the scribe, “I have found the Book of the Law in the house of the LORD!” And he gave it to Shaphan, who read it.”— 2 Kings 22:8 (BSB)