I've seen tennis ball launchers that use dry ice, but could you use liquid Nitrogen, and if so, where could I buy it?

Chemical supply houses or ice factories, usually. The actual liquid nitrogen is pretty cheap but you need to put a deposit on the special dewar flask used to carry it, which can be a bit pricey. Also, it's going to be very hard to use in a potato gun.

It'd be like a regular dry ice bomb. Put some in a plastic bottle and cap it. Drop it down the tube, drop the tennis ball, run like hell.

Don't do that.

Wouldn't that....you know...damage the launcher?

I mean....if that actually works, why not send it flying with a good old HCl bomb (DON'T!!...though I might try for shits and giggles)

what the fuck did they do to this site its was okay now it looks gay but yea i dont know about the damaging the launcher

The new look is...unique lol

It's got it's flaws, but it still works good.

Just a side question, sense we are on the topic of lauchers (kinda) what would be a good material? I mean like a good "light-heavy" explosive kind?

what do you mean 'light-heavy'?

if you are looking for propellant for a launcher it depends on the
weight and size of your projectile.

in a normal spud cannon i use car 'jump start' as it is super flamable
and easy to get.

I know that schools often have some liquid nitrogen. If you are still in school you can ask a chem teacher for some.

You could make a sort of cannon for the launcher because the ball will just slowly be bushed up and out, pressure needs to be built up in it some how... maby if you put a slide that you could slide out when you wanted it to fire. idk its a neat idea I think Im going to try to make 1 myself.