Another option is made by the Dixie slug company of Old Town, Fl. They make a buckshot load called the tri-ball load. It utilizes 3 hardened lead balls that measure .600" in diameter. Compared to the anemic results that are produced by 00 buck, the tri-ball hits with almost 2,000ftlbs of energy at 50 yards. If I was forced to use buckshot, this is the load I would use.
http://www.dixieslugs.com/products.html