There are several links and a bibliography at the following website OASIS NYC. They are mostly for resources in New York, but it is a good starting point.

Also a search for “community garden” at USDA returns 254 hits. The same search at Agriculture and Agri-food Canada retrieves 36 hits for community garden (or community gardens); and - Ontario homepage gets 125 hits for community gardens.

additional suggestion:
I'd recommend getting in touch with the Times-Up Community Gardens group in NYC. They are very active and may have expertise on what other resources are out there. One of their organizers is a librarian, btw.