I've worked with co-ops both on the retail grocery side and ag aggregator side, and the traditional supply chain is similar. The ag co-ops serve as a middleman, and the retail grocery will usually deal with a distributor.

The grocery co-op had way more direct accounts (hundreds) than a traditional grocery, but that was mostly for smaller company specialty goods. The vast majority of the product moving off the shelves was bought from one of two big distributors.