There are several channels

1. Wheat merchants are wheat bosses. They obtain wheat from various channels and then resell it

2. Futures, this is usually done by very large flour mills

3. Farmers, some small flour mills in rural areas, collect grain directly from farmers. The disadvantage is that the quality of wheat products is unstable. If you want to produce special flour, it will be a bit difficult

4. National turnover grain, grain is a strategic material. Generally, the national treasury purchases some wheat every year. After two or three years, the country will sell these grains (if the grain is put back, it will spoil), usually in the form of auction. The quality of this kind of wheat is slightly inferior, but it is cheap and many flour mills will buy it.