There are a number of places to look for used barrels. Classified ads section of hunting forums are good (here on PM, over at Accurate Reloading, Graybeards), the gun auction sites (auction arms, gunbroker), plus eBay. There are several dealers that carry them also. I understand that the best is Ed's Contenders. I don't know his website off-hand, but an internet search should bring it up.