Go to a neighbor's house that has the Kiddie furniture series, the set with the floor that looks like puzzle pieces. Find the long dresser that takes up two squares and press A (or tap yourself) while facing it to open the drawer. A message will appear saying "You found 10 rupees! Lucky! Too bad you can't use them in this town."
There is a triforce on the back of Katrina's tent.
Sometimes Tom Nook and Redd will sell item from the series, such as the Triforce and Master Sword. Sometimes neighbors will also have these items. Give them shells and talk to them until they give it to you.