Fish in the ocean during summer
Fish in the ocean on a sunny day between 12:00 PM and 4:00 PM
Fish off the beach or from the Ginger Island western shore
Buy one from Krobus on Wednesdays if he is selling it
Use a Magic Bait to catch one in any season or weather during the required time
Check the Traveling Cart when it appears for a possible sale
Catch one from fish ponds only if you already have a pufferfish population to harvest from