I have not yet found a source for cotton quilting-type fabrics that are made in the US. I have found solid-color blends, but that's about it.
Yes, cotton is grown here in the US but I think it is more likely to wind up in denim and apparel fabrics, rather than fabrics for quilting. Apparel is, after all, a much larger market. Everybody wears clothes (well, ALMOST everybody!) but not so many people sew or quilt.
If you want to know more about the cotton industry, here are two reliable sources:
http://www.cotton.org
http://www.cottoninc.com