"Neither he nor Frodo knew anything of the great slave-worked fields away south in this wide realm, beyond the fumes of the Mountain by the dark sad waters of Lake Núrnen; nor of the great roads that ran away east and south to tributary lands, from which the soldiers of the Tower brought long waggon-trains of goods and booty and fresh slaves. Here in the northward regions were the mines and forges, and the musterings of long-planned war; and here the Dark Power, moving its armies like pieces on the board was gathering them together."

But yes, cannibalism and human forage is not sustainable, but might stretch a 10 day supply to 15.

(There may also have been farms in the East that I'm not remembering. The East was definitely under Sauron's thumb, although perhaps not so firmly as he would have liked.)