This is a quick dash, but here are a couple of sources to start:
– Mother Earth News has a few posts that list sustainable seed companies. This is just one of them.
– The Seed Savers Exchange is a national network devoted to growing out and sharing (for a small cost) open-pollinated vegetable, herb, and flower seeds
Here are three of my faves:
– High Mowing Seeds
– Prairie Road Organic
– Johnny’s Seeds