To find agricultural planets (and I've never visited an agri planet that did not have any fish in stock): Go to the Galaxy map. Click on the "View" tab. Then click on the "Map" sub-tab. Now click on "Show colour as" and select "economy" from the options. Agricultural planets are bright green. Since some systems have Agriculture as a secondary economy only, de-select all the options except "Agricultural". Your starmap should now show only green stars, and all of those should have fish in them somewhere; find an Agricultural station in that system and go there for your fish.
To find where specific commodities are coming to and from using the trade map: Go to the Galaxy map. Click on the "View" tab, then "Map". Scroll down to the Trade section and make sure "show trade routes" is selected. Now de-select all the buttons on the right-hand-side except "Foods". Now press the little > arrow next to "Food" and you should see a list of things that include "fish". De-select everything except "Fish". All of the curved trade lines now showing should only be green fish-lines. Find a star where the fish-lines appear to be radiating outwards from, and there will be a planet somewhere in that star system that is exporting fish. It will be an Agricultural station there, since fish never appear for sale anywhere except agricultural planets.