As far as I know, there is no fishing in the game. Kind of a shame, honestly, it would have been a nice addition to the game. A little peace and quiet, without something trying to kill you every few dozen steps.
Agatha, in the town of Graycott, sells raw fish.
Some of the tougher monsters (nightmares, Minotaurs, Drakes) will randomly drop fish as their loot.
You can occasionally find fish as loot in various barrels and crates.
PS: If you have no money, you can sell things like cloth, ores, wood, and pretty much anything else you can harvest. Barnabas (in the town as well) will buy things like ores, wood, leather, your old armors, and cloth and Agatha will buy things like wheat or anything you have cooked.
You can also sell your babies to Agatha, if you are desperate for cash.