I've shot Dixie cups filled with paintballs out of my air cannon, and they seem to work fairly well. Just cut several slots down the side of the cup so that it will catch the air and fall away upon exiting the barrel (you can expiriment with the number of slots that you cut to find what works best for you; the less you cut will likely increase the distance before the cup falls away and give you a tighter pattern but increase the risk of injury to someone that gets hit with it), fill it full of paint and slide it down the barrel. Stuff a few paper towels down on top of it for wadding and you're good to go. Toilet paper tubes cut in similar fashion may work as well.
I don't know if this will work in your situation or not since it will depend on the diameter of the cannon barrel, the number of paintballs you are trying to shoot, and your desired range and pattern. I haven't tested it thoroughly, but it was pretty cool for shooting trees in the back yard
