Patagonia makes a good hemp blend and Tentree has some nice stuff too (they also have organic cotton of good quality), but mostly you just have to hunt for them. I tend to thrift mine so they often don't have tags, but another common place to find them is little hippie clothing shops and fair-trade type stores. Hemp in particular is very durable and what jeans often used to be made out of!

If you are looking for the most ecologically sound - avoid bamboo. And read up on it. The process is not very friendly and bamboo plantations are not either currently. Bamboo fabric is a type of "viscose rayon" which is made by dissolving the cellulose in the bamboo and then extruding it. I'd avoid anything rayon or viscose if that is your goal in fact, including modal. Tencel Lyocell is the best of this family since it uses much safer and recyclable solvents.