We work with a broker at the Chelsea produce market in Chelsea, MA. Twice a week, I email him an order for the produce we need from the national market, specifying details about each product (size, grower, production region). He uses his knowledge and relationships to buy the best product he can, from the large wholesalers there. The market has around 240 dock bays, and is open from 2 am to noon. We send a truck down to arrive at 8 am, and pickup the produce from the wholesalers where our broker ordered.
The overall produce market is structured into growers, that grow and harvest the crops. Packers, that purchase crops from the growers, wash, cool and pack them, and shippers, that purchase from packers. The wholesalers in Boston purchase from shippers, and sell to regional retail and wholesale customers. Many companies are vertically integrated, and own multiple links in this chain. Larger groceries and supermarkets either negotiate directly with the packers, or the wholesalers at the Chelsea market. A large store can commit to buying on a large enough scale to drive their retail price below what I can pay wholesale during the peak season for a commodity.