Where can I get a Mermaid Pendant?

To acquire the Mermaid's Pendant the player will need to go through some additional steps in order for it to become available. The first step requires the player to raise a character's relationship level to ten hearts. The second step is that the player must give the marriage candidate a Bouquet of Flowers in order to establish that they're in a committed relationship with them. Following that, that the player will receive a letter from Lewis telling them that if they wish to marry, they should seek out the Mermaid's Pendant. The fourth and final step comes in four parts:

• Player must have upgraded their house at least once.
• It has to be raining out.
• Player needs 5,000 gold minimum to purchase the Pendant.
• Player repaired the bridge by the Beach with 300 wood. Once all of those steps have been completed, the player will be able to purchase the pendant from the Old Mariner following the next available rainy day.

Source