Ever heard of People Tree?
It's a fair trade / sustainable clothing website for women and men. They also have a few accessories as well. It's great for those avoiding synthetic fabrics such as myself.
Their search feature is useless (not uncommon, unfortunately). I searched "100% cotton" and got nothing. Same as my "cotton" search. They have a lot of great cotton items on their site, though - I promise! You just have to weed through the items.