ModCloth is undiscriminating when it comes to sourcing, although I think I read somewhere that they prefer independent designers over well-known labels. I discovered several months ago that you can narrow your search by country of origin or production designation by typing it in the search box. They offer plenty of domestically produced items as well as a small selection of fair trade dresses. They also have a section dedicated to vintage clothing, which, by virtue of being second hand, are perhaps the most ethical garments to buy.
Made in USA:
ModCloth has become my go-to place to browse because there are so many options for the conscientious consumer.
*Click each set to be redirected to polyvore for links to each item.