Mystery boxes can be obtained from the Quiz Master random event. Players can choose between 1,000 coins or the mystery box upon successfully completing the event. Multiple Mystery Boxes can be owned at once, and they can be banked.
Puzzle box - OSRS Wiki
Puzzle box - OSRS Wiki
Seed box - OSRS Wiki, Seed Box
Arceuus bank - OSRS Wiki
Mystic cards - OSRS Wiki
Treasure Trails/Guide/Puzzle boxes - OSRS Wiki
Store – BoomScape
Quiz Master - OSRS Wiki
Random event gift, RuneScape Wiki
Welcome to the Old School Bot Wiki! - Old School Bot
Legendary Mystery Box! - Media - RuneWild
OSRS Murder Mystery - RuneScape Guide - RuneHQ
Present - OSRS Wiki