The only reliable way to get a new good is to get them through missions. Other than that, converting a settlement to a city is supposed to remove food resources like grain. Either way, cities are far less efficient at generating surplus goods. You also have to forego other, usually more important resources that a city generates like research.
If you are very small, then I would just suggest stacking import slots to the city you want to grow. If you're large, then you should be swimmining in grain already...