I built an 1857 12-Pounder Napoleon for my front yard last year. I did purchase the barrel (a fiberglass replica) from WTLTH, LC, in Osprey, FL. It wasn't cheap, but they did a very 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.
For the wheels, Amish wagon makers.
Good luck. It was quite an enjoyable project!