I have purchased ecology cloth from Bellwether Dry Goods and have been thrilled with the quality. It is the perfect muslin if you want your quilt to have the "old feel" after quilting and washing.
I first purchased from them at a quilt show in the DC area, but have called them and ordered from them directly. They are wonderful people and their product and service is second to none.
http://www.bellwetherdrygoods.com/