If you are in the US, Central Milling flour. Central milling is the supplier for some of the top bakers/bakeries in the nation, including Tartine, Josey Baker, Acme Bread. They also produce pizza flour for Tony Gemignani.
The flour is distributed through Keith Giusto Bakery Supply in Petaluma, California. The Giustos are actually in partnership with the farmers, so they are involved with every aspect of the cultivation and milling.
While they primarily serve the trade, they repackage some of their most popular flours into 5 pound bags for the home baker.
Since I live in the Napa Valley I buy their flours at their store, so I have no idea what shipping costs are for their flours.
I regularly stock their High Mountain, High Gluten, Artisan Craft, Beehive, and Pastry flours.
The High Mountain High Gluten is of course high protein flour for bread.
Artisan Baker’s Craft is 11.5% protein and unmalted. It’s an excellent all purpose flour for baguettes, laminated doughs, and cookies like chocolate chip.
Artisan Baker’s Craft Plus is malted.
Beehive is 10.5% protein well suited for cakes, muffins, quick breads, and cookies like shortbread and sugar cookies.
Pastry flour is self-explanatory.
Although it doesn’t show up on their website for purchase, they repackage low diastatic malt powder for the home baker. I currently have some of their diastatic malt, spelled, and rye flour in my pantry as well.
Keith Giusto and his nephew Nick are both exceptional bakers. Nick Giusto was on the US team for the Coupe du Monde de la Boulangerie. So they both know flour from the baker’s perspective.