Before you try substitutes you need to understand some things that are being learned about them.
First in their standard use in cartridges and muzzle loading firearms, substitutes generate higher presures than the black powder loads they replace. Some of the substitutes generate substantial pressure increases. I am unaware of any pressure work done on cannon size loads with the substitutes. Contact the powder manufacturers of the substitutes for recommended loads before you use them.
If you use substitutes in small cannons in calibers for which loads are published, substantially reduce the starting loads and work up. The loads on the charts are for hand held weapons. Cannons are not hand held and can be come secondary projectiles if fired with the loads on the charts. This advice applies to blackpowder loads also.
Pyrodex requires a resistance to work properly. Tight wad or crimp. Tight fit in large cannons raise pressure and projectiles are difficult to properly load. Windage is strongly recommended in large cannons.
All the the substitutes are smokeless powders. All the substitutes are corrosive. If you use them you must clean aggressively to remove the corrosive residue they leave.
Black powder is available from several suppliers in direct shipment in 5 lb lots. Contact them for details. When you look you will find the Powder Inc. seems to have the highest prices. Powder Inc prices include shipping and haz mat fee. Others list their price per pound but when you add in the shipping and haz mat fee their price per pound is pretty much the same as Powder Inc. I think if you contact the folks listed below you will find most of them now offer the 5 lb shipment.
Powder