In game, you can go to the galactic map and search by specific commodities. For example, you'd find the sub category of food and select the Fish checkbox. Colored arrows will show you which systems produce it, and where they ship it based on the direction of the animation on screen. The important thing to remember is that the galactic map shows if the system exports a particular commodity. If the targeted system has multiple stations, not all of them might be selling what you're looking for so don't just head to the closest one. Take a look at the system map and click on each station and see which one is producing the item you're looking for or something similar. Like Kandarus said, for fish, look for stations orbiting Earth-like planets. They'll usually have an assortment of foodstuffs from fish to animal meats to grain and fruits and vegetable systems. Keep a mental note of which systems have what and you'll be able to work out viable trade routes.