They are some of my favorites. For American sours you can't beat Russian River in Santa Rosa, CA. They get a lot of recognition for their Pliny the Elder (and the Younger), but their Belgian style beers, especially their sours, are phenomenal. If you can get them, Temptation (sour aged in chardonnay barrels), Supplication (sour aged with cherries) and Consecration (sour aged in Pinot Noir barrels with currants) are some of the best beers I have ever tasted. New Glarus in Wisconsin also have some great sours, but they are even harder to get your hands on.
Karl