I baked a lot with Anson Mills flours, but would love to know of other mills that ship heritage grain that I could experiment with for my sourdough!
In the Pacific Northwest, I use:
Bluebird Grain Farms
Camas Country Mill
I usually mill my own flour with a Mock Mill (I highly recommend one!) from the grain berries, but all of these companies sell flour. I usually source my rye and ancient wheat berries from Bluebird. Cairnspring Mills and Camas Country are loosely affiliated, and Cairnspring is just around the corner (they may even share a back fenceline) from the WSU Bread Lab and has a strong relationship with local growers for both heritage and new grain varieties. Their Yecora Rojo Type 85 is a great flour for all types of artisan breads. The Edison Organic is also excellent. You'd be happy with both flours for your sourdoughs--I know I am! I've toured their mill a few times and it's top-notch. Tartine Manufactory uses a lot of Cairnspring flour. For easier to find and lower-priced freshly milled flours, Wheat Montana has some great products. Bake on!
I thought of another flour to suggest although I don't yet have personal experience using it: Farmer Ground Flour in NY. Here's the link: