I've been hanging back on this one, but I'll speak up now and see if I get corrected.
I own four cannons, one a 3 inch Parrot rifle replica.
I have a 54. Cal Hawken rifle,
I got rid of the navy replica 36 Cal.
For the most part you can't ship Black power here because it is considered an explosive and nobody will ship it.
Want to buy your own boat? You can bring over all you want.
For that reason everybody uses Hogden which is not considered an explosive.
One pound of powder isn't squat for a cannon. For a cannon you want a very coarse powder, Rilfles finer,
and pistols finer still. Check for Hogden products.
If you really, really have to have traditional Black powder, your best bet is to make your own as you can
easily obtain the chemicals, lead balls, and mixer to make "Meal powder" in whatever grain size you want.
Government won't know you have it, it is legal, and you can also make some really
awesome fireworks if you are so inclined, although I don't recommend anybody to break the law.
There are lots of Youtube videos out there on the powder stuff.