If you're in the Portland area, Borealis (a bakery on Ocean Avenue) sells Maine whole wheat flour by the 5 lb bag; there's also at least one vendor at the Wednesday farmer's market who sells locally grown grains, at least during the summer.
I would contact either individual co ops in Maine that sell locally grown grain products (there's one in Belfast and one in Portland, as you probably know). Or you contact the co op network through this site: