Lost Abbey, the Bruery, and Upland are all breweries that make sours and there are a lot of others, but they may be hard to find as well. Goose Island has done some also. As far as bigger breweries or easier to find stuff goes, New Belgium has put out several-La Folie and La Terroir are fairly common. Kick (cranberry/pumpkin sour) was a seasonal this year and I don't know if they are redoing it or if it was a one time thing. Clutch was an imperial stout blended with 25% sour, and brewed with the band Clutch, so again I don't know if that will continue to be produced. They are also opening a new brewery in NC, so if you don't get their stuff now, you probably will after that.