Traditions offer small cannons and cannon kits.
http://www.traditionsfirearms.com/index.php?cPath=4_59
The Pocket Cannon (no cracks):
http://www.pocketcannons.com/
Pocket artillary
http://www.pocketartillery.com/
For large scale try Hern Iron works:
http://www.hernironworks.com/cannons.html