They probably use one of the 'big box' purveyors, as mentioned above. The pricing schema is a very negotiated and complex part of the business. Often, there are pricing incentives to the parent company (above the unit level) that may include rebates, volume purchase agreements and spec alignment. Price, at the unit level, does not always make sense, but will often have some ceilings attached, so that when you could be spending $60 on a case of tomatoes in February, the price may be locked.
Best of luck with your new venture!