See the Food Research and Action Center and the Coalition on Human Needs. FRAC and CHN are effective advocates for low-income Americans, and both organizations cover food assistance programs, like food stamps and WIC, quite well.
The School Nutrition Association (formerly the American School Food Service Association) has for decades been a leading voice for child nutrition programs, like the National School Lunch Program and the School Breakfast Program, and also for the interests of school program managers and employees -- and, for the moment, its website appears not to have any misleading food advertisements.
The Center on Budget and Policy Priorities, in addition to being an equally effective advocate for the non-rich, has paradoxically managed to maintain a reputation as an even-handed and authoritative source of timely information, to such an extent that its reports are widely read by conservatives as well as liberals. For example, the Center's position can never be classified as tax-and-spend liberalism, because in recent years it has represented stricter fiscal responsibility than the Administration position.