The key can be purchased at the item shop in Wellgarth by selecting the blank line within the shopkeeper's list of available items. The fact that the shop sells the key is hinted by a townsperson in Wellgarth. Unlike the other keys, the Jailers Key can be purchased multiple times and sold at shops like any normal item.
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5 Seals ( Star Moon Sun Water Life) • Cloak of Wind • Evil Statue • Golden Key • Jailers Key • Leaf of the World Tree • Lar Mirror • Moon Shard • Rubiss Charm • Ship • Silver Key • Teleportal • Watergate Key