Did you get to see the Andrew Wyeth Retrospective at the Whitney Museum
in NYC? I had the opportunity to see it about a month ago and have a
new appreciation for his work. The Whitney may be able to provide you
with some good leads, also I am remembering a museum in PA, Brandywine
or Chadds Ford ??, which has a collection of Wyeth paintings... I don't
remember the name, but perhaps searching for that may help too. Good
luck with your research and writing!