Julia Sherman profiled a wig maker. The basics:
The market for human hair is generally limited to places with impoverished populations willing to sell a two-foot ponytail -- the product of two years of growth -- for twenty dollars. Dark hair comes primarily from South America, India, and Mongolia. Helene says that the ample selection of hair colors and textures in South America -- the result of more than twenty-five generations of intermarriage between Europeans and indigenous people -- make it the ideal source region.
(Hat tip: Kottke)