You might want to try you local cooperative extension service. They're usually backed by the state's agricultural universities and they also train master gardeners, which can be a huge resource of info. Also, any organic farmers or their type of publications in your state may have some info on some native species that have slipped into cultivation. I would definetly go after the master gardeners and local extension service first. And ask the extension agents what books they remember from college concerning the state's native flora.