Building a mountain bike with American made parts is more and more difficult each year. Fortunately there are a few companies out there that are keeping manufacturing in the USA like Cane Creek and Thomson. One of these companies is Hayes Bicycle Group with their Wheelsmith spoke company. SickLines has a great article on Wheelsmith and their Wisconsin based spoke manufacturing facility. The article has an interview video and great photos of the spoke making machines. They actually even bought spoke making technology in Japan and brought it back to the states.
A good place to purchase Wheelsmith spokes is through your local bike shop or online through Jenson USA.