I know that many of the Chards in Napa buy grapes from the Central Valley for added sweetness. Most of the Central Valley, however, is too damn hot to grow anything more than that Thompson seedless crap and some poor quality high yield vinifera, most of which is processed by either Gallo or Franzia, who are related by marriage, btw.
You are most welcome to visit Bogle in Clarksburg for what are the most amazing reds outside of Burgundy and I also invite you to visit Spinetta in Plymouth (near Placerville) where the 'Fun & Yummy' Zinfandel has made fans of a number of conservative oenologists.