The darts for the minigame in Lost Judgment can be found at the below locations!
Standard Darts
You can purchase the Standard Darts from The Bee (Darts Shop) in Ijincho.
Miracle Darts
The Miracle Darts can be obtained from the squirrel graffiti challenge. Before you can collect these darts, you must complete the Squirrels Just Wanna Have Fun side case. Once you’ve done it, you’ll then be able to go around the towns examining squirrel graffiti then mimicking what the squirrel is doing to obtain all kinds of goodies including these darts. For these darts in particular, head to the location shown below in the Bar District.
Look for the squirrel squirrel with headphones in the park next to Survive Bar.
Press L3 to examine it then switch to the Noise Amplifier. Follow the signal to the base of the tree to retrieve a bow. Futaba will show up and you’ll hand over the bow to receive Miracle Darts.