L Bar M Ranch: http://www.lbarmranch.com/ (their dog kennels are awesome and are probably the strongest dog kennels on the market)
Options Plus: http://www.optionsplus.com/
Billet Barn & Corral: http://www.billetbarns.com/index.html
TK Products LLC: http://www.tkproductsllc.com/index.html
All in all, L Bar M Ranch is your best bet. Don't rely on those store bought kennels from places like Wal-Mart or any pet store to hold a dog in because most of the time, they won't. Store bought kennels don't handle all types of weather very well and tend to damage easily. A dog can easily chew through a store bought kennel, too. Good dog kennels like L Bar M Ranch are very expensive, but worth every penny. The L Bar M Ranch crew are also known to be extremely nice people and in many cases will come to your house and set up the kennels that you ordered from them so that you don't have to set up them.