A holder of an alcoholic liquor wholesale license shall purchase and import all alcoholic liquor from a primary American source of supply. For purposes of this section, primary American source of supply shall mean the manufacturer, the owner of alcoholic liquor at the time it becomes a marketable product, or the manufacturer's or owner's agent, who, if such liquor cannot be secured directly from such manufacturer or owner by American wholesalers, is the source closest to such manufacturer or owner in the channel of commerce from which the product can be secured by American wholesalers.