From the Patagonia site, under the Common Threads tab:
Worn out Patagonia Capilene baselayers, Patagonia cotton t-shirts, all Patagonia fleece and Polartec fleece from other manufacturers are now recyclable through Patagonia’s Common Threads Recycling Program.
Drop off your tired old garments at a Patagonia store, participating Patagonia dealer or send to the Patagonia service center, ATTN: Common Threads Recycling Program, 8550 White Fir St., Reno, NV 89523. They’ll recycle it.
Also, American Apparel is a store that offers clothing in 100% organic cotton. Check it out: