I had built an 1857 12-Pounder Napoleon for my front yard a couple of years ago. I did purchase the barrel (a fiberglass replica) from WTLTH, LC, in Osprey, FL. It wasn't cheap, but they did a very good job, and included copies of the blueprints for the carriage. Their website is located at: www.GreatGuns.org

There are other barrel manufacturers to be found on the internet, including those that manufacture "working" barrels.

For the wheels, Amish wagon makers.