Where in the Bay Area can I buy natural-fiber underwear and socks?

Doesn't American Apparel sell organic underwear?

Patagonia fits the bill, and they sell underwear and socks.

American Apparel "Organics" has briefs on sale right now. I have some that I've been planning on silkscreening and they are super cute. http://store.americanapparel.net/women-organics.html
Patagonia's briefs are in a nylon/spandex blend, but they do have a cute black thong made of recycled polyester.
I've been meaning to check out the new Sustainable line at Nordstrom. Free returns and the organic cottom undies are adorable. Plus, I know someone who does compliance work for the Nordstrom line and visits most of the lingerie factories abroad. Women employees are given maternity leave and there are even scholarship programs for many workers that live close enough to a university to attend classes. Nordstrom isn't perfect by any means, but I do trust that they are making improvements to be a more ethical company. Buying this product might encourage them to use more organic cotton in the rest of their clothing lines.

Whole Foods.

There is a company called Wintersilks www.wintersilks.com that offers an extensive line of natural fiber clothing including underwear. Not sure about their trading practices though—I suspect most of their items come from China.

yea.. American Apparel has these. Plus everything they sell there is made in the US (specifically downtown LA i think). they dont use overseas sweatshops.