I love Cannons.. They are real fun.. But can be a problem..
-- The 1st thing I would do is start Small.. Get a small cannon to play with..
-- Then Build up to another larger cannon that you feel comfortable with it..
-- Always treat the cannon as a possible bomb..
-- Always under load it to its maximum powder load..
Purchase your cannons for someone who has been in building them for a while
One place that I have found to have a nice cannon selection is
A lot of variety and can build custom work.. Plus I have trusted the guy with my own fingers and eyes.. But there are other great cannon builders So do not stop there..