Fish is a commodity which can be obtained in a few different ways. The quickest way to obtain fish if you have a ship is to take a fishing net item out with you while sailing and collect the bounty of the seas with it. You can also travel on a trawler which has its own nets to gather fish in larger quantities. An alternative if you don't have a ship is to get a fishing rod item and go fishing from the banks of a river or lake. You will catch individual fish which you can then convert into the fish commodities once you've caught enough of them. Fishing rods can also be used on board ships though they are not as fast at gathering fish commodities as the fishing net.