Fish and seaweed can be produced in the fish farms and/or ordered from suppliers.
To produce fish and seaweed by yourself, tap on the sea dock to check what requirements you have to unlock before you can unlock the fish farms. Check the dialog windows to know the next steps!
**Make sure that your Nigiri stand is at level 3, and do the Sake Nigiri research. Your fish farm zone will be unlocked after those actions.
Once you can open a fish farm sector, you can choose which good you want to produce in each sector according to your preferences. The same happens in your garden plots!
You can also buy fish and seaweed from providers if you don't produce enough.
Don't forget to do some research to unlock the products desired!