Ah, that's right - you need it for the fishing quest. I suggest using the fishing spell "reveal fish school" while in Cyclops Lane to locate the myth fish. Then only try to catch the myth fish (light yellow). If you don't have that spell, you can get it from Lucky Hookine in the commons. If you don't find any, refresh the pond with the refresh spell and try again. It might take a little bit, but you will find one.
For those who can't get to Zafaria yet, yes you can catch the fish sandwich in Cyclops Lane. I had better luck trying in the stream side rather than in the large pond. If you can get the spell, winnow myth fish, which banishes all fish except myth ones, that helps. If not, use the reveal school spell and try to chase away the non yellow fish. I think you can also go to Castle tours and look for a Myth school house to fish at. I think the fish sandwich might also be found there.
Go to Cyclops lane..the stream behind the yellow tent closest to the cyclops. That's where I catch them. Also if you go to castle tours and go to a myth house you can catch plenty there. Good luck. Have fun.
The fish sandwich is commonly found in Cyclops Lane, towards Festival Park. The fish is a Myth fish, so I would recommend ( to get the highest possibility of getting / catching the fish ) is to purchase a myth lure ( "Lesser Fable Lure" ) from Lucky Hookline, along with his "Reveal Fish School" spell, since it reveals the magic school of all the fish within a reasonable distance from you. If you cast the Reveal Fish School spell, and use a myth lure, you're bound to catch a Fish Sandwich eventually! If you find yourself having difficulty catching it even after this, try server hopping, or buying a fishing luck elixir from the crowns shop