The first six items from Majora’s Mask you could find in the Great Spell War.
Ocarina of Time: Link is given the Ocarina of Time by Zelda as he is leaving Hyrule in the prologue of Majora’s Mask. She also teaches him the Song of Time, warning him to play it in times of need. Link's ability to travel in time by playing the Song of Time is the only thing that allows the entire story of Majora’s Mask to take place. Without it, Skullkid would have succeeded in their apocalyptic plan of pulling the moon down from the sky. For continuity purposes, Majora's Mask takes place in the child Link timeline after he is successful in defeating Ganondorf.
Legends tell of a vast store of ancient arcano-technology held in a hidden vault behind a sealed gate that can only be opened after the fabled Horn of Doom is blown. A thousand years of peace were shattered when foolish adventurers sounded the ancient horn without knowing it would warn all the world that the vault would soon open. After the warning, the great powers of the world quickly mobilized their vast forces to ensure they would be the first to find and secure the vault.