Equipment can be bought in shops, dropped by monsters, or given as rewards for completing quests. Monsters will sometimes drop rare loot.
To obtain rare loot you need to use either an Iron Key or Mithril Key. Another option for opening the chest is to pay 150 Tokens, wich is expensive considering an iron key only costs 25 Tokens.
The free but not safe option to get the item, is to picklock it. This chance can only be increased by a character with the single highest Luck in your party, preferably a Thief (53.4% Luck maximum).
At rare drops of bosses you can only use Mithril Keys.