One of the best teacher resources is simply a map! It's easy to talk to your kids about what kinds of crops are grown in your own area of the country, and then what kinds of crops are grown in other areas. For example, if you live in the Great Plains, wheat is surely on the list of top crops, and if you live in New England, cranberries are produced in abundance. You could even read some books about this kind of thing, just to drive the point home that different things are grown in different areas. Whether or not a certain crop will thrive depends on the temperature, amount of rain, type of soil, etc.
If you'd like to expand this and learn more about each state, here are some geography games to help. It may be helpful to check out library books about different states and learn a little about them, including the food crops that are grown. Here's an online resource from the Environmental Protection Agency (EPA) that can give you some background on which crops are grown where. The point is to open a dialogue with your kids about where their food is grown, and just supplement that discussion with a few other resources as you see fit. Discover the answers together and make food geography part of your nutrition curriculum!