I can't tell you where they source their material, but Fox River Mills socks are made in Osage, Iowa and carried by quite a few retailers. Their annual sock sale is a local "event" and people come from a long ways to buy socks. I wear their steel-toe socks in my work boots. They're thick and comfy. 

This should be the dealer locator. I know Mills Fleet Farm carries some of their line. https://www.foxsox.com/apps/store-locator