Here are some propellants that can be used for combustion cannons.  Generally hairspray is the easiest and somewhat safest propellant to use; try to stick to hairspray.  Propane is probably second best.

 

Hair Spray:	The most basic and most commonly used propellant is probably hair spray. All you have to do is just open up your chamber, spray a little in, close it up and hit the sparker. You need to use the aerosol type of hairspray, not the pump-action. This propellant is fairly good, but it isn't as powerful as others, and it might make your gun sticky. Note that hair spray only works well were it is hot and humid. You will notice that in cold your performance of your propellant will dramatically reduce.
Propane:	Propane is actually a commonly used propellant. It tends to give a good powerful shot without being a dangerous propellant.  Propane is cheap, basically safe and easily dispensed. It is very cheap, as one bottle will give at least 1000 rounds from a typical launcher.
Carburetor Cleaner/Starting Fluid/Butane:	Butane appears to be more powerful than propane however I am not sure. A lot of people I know uses these. They should be easier to dispense than butane, and won't gum up your cannon. Butane is powerful and can be dispensed into a pinhole but you can easily put in too much, or too little.

If you have any more to add, E-Mail me and tell me all about it. I'll put it on the page with the rest of them.