The answer: a lot of places, actually. But I'm particularly fond of one mill based in the middle of nowhere: Harvey, North Dakota's Dakota Prairie.
I love that they sell retail quantities of their amazing, finely ground gluten-free flours direct to consumers.
They mill all their gluten-free flours in a separate, certified( GFCO) gluten-free mill. There's no fear of cross-contamination. Most of the flours they mill are organic, though you can buy some conventional flours (but why would you?). All are GM-free.
I love their buckwheat, millet, rice, quinoa and sorghum flours. Their brown rice flour is lovely.
(and ps...I'm not shilling here. All of my GF flours are from these guys. The quality is remarkable).