Proposed Canon
Because Farms cause provisions to pop in nearby cities and Natural Allies have no cities, it's not clear how they would secure provisions. It's possible that the farming that Natural Allies do for money is unrelated to Farms used by cities. It's also possible that they get a share of the provisions that pop in some way -- for instance, the Gobwins of Gobwin Knob might receive a share of all farmed provisions that would otherwise pop there, so "farming" just means allying themselves with a Side that has farms, and stationing units in their cities. it is also possible food type units randomly spawn in hexes like combat units do.