Amended yet again 10/10/09 (older links are defunct).
A great resource for smaller quantities of organic sustainable fibers and supplies is NearSea Naturals. Handily enough, they’re based here in New Mexico.
Another option is Harmony Art which features certified organic cotton fabrics printed with eco-friendly inks and dyes.
For larger quantities of sustainable fabrics, both the fabric sourcing and the sustainable section in the forum are useful. I regret it can’t be public but people are cautious sharing information publicly that constitutes competitive advantage.