You could join the gorilla gardening movement, let me explain.
Urban areas seem to hold space where no one is making use of it, what many call brown sites. Try gaining a little knowledge of growing veg, plenty of good books in your library. Then start small, say plant a few rows of carrots between the rows of flowers in your local parks flowerbeds. Keep it small and spread your crop just in case its found. These flowerbeds tend to be the best as they have been well fed over the years.
As you gain in confidence and knowledge spread your crop to other areas and other types of crop. Your planting needs to be done covertly and left unvisited until harvest to stave off detection.
I first found out about gorilla gardening whilst living in London 20yrs ago, anywhere that held a little soil was holding squatters of the food kind. Since then i have found i have green fingers and seem to get a good return for my efforts.
HOWEVER BE WARNED
In looking for out of the way sites i have found a few places holding a harvest of cannibis plants. The choice is yours but i leave these places as i find them and dont return, be my luck a copper turns up and nicks me for cultivation.
I know this isnt like finding ready grown food stuffs but combine your visits to the library to learn knowledge on growing and identification
Please bare with me in my ramblings, I'm an ageing hippy struggling to control the voices in my head.