I'm never 100% sure if FNB dives for any of its food, but it is almost always food that has been diverted from the waste stream, and you're bound to find freegans and allies there. Plus, nobody else manages to make such good meals so quickly for so many so consistently as this group. If this appeals to you after checking it out a few times, consider volunteering. It's a great group.
Fridays they meet in Boston Common, usually across the street from St. Paul's Cathedral, who lets the group borrow their tables.
I will rarely be there, as my work doesn't really allow for it.