Launchers are not generally mentioned as allowed on normal games. But you can always ask. The iffy point is that depending on the shell you can fire more than one ball per shot, but compared to a stream of paint from a fast trigger they are nothing. If you ask nicely sites may let you use it, and you can offer to load with one ball at a time. They are more visible at scenario games.
They make a very satisfying noise when fired so can be used empty for affect as well. Various types of shell are available for BBs, .43" paintballs and .68" paintballs, plus you can put in other things such as powder for clouds or if the launcher has a long barrel then you can leave the shell empty and launch something like a sabot or nerf rocket. (Depending on game rules)
Getting them can be pricey. In the UK they are less common. RAP 4 is the basic one for .68" shells, but just about any airsoft launcher will do and can be got hold of pretty cheaply from Hong Kong etc.
The thing to be careful with airsoft ones is any barrel mount will be real life size and won't fit a paintball barrel without adjustment. M203 types that fit by rail won't ba a problem - as long as you have the rail.
Importing can be tricky as you have to get past customs - I got mself an airsoft AK launcher, if I was a registered airsofter I could have just ticked the right box. As I'm not I had to go trough some rigmarole of explaining why I was importing it. I have since seen the case of a guy getting a marker refused entry so it can depend on the opinion of the customs guys - or a policy change.
If it's still available there was a QLoader for sale as linked below, it's with A5 fittings but you can discard or offer them out if you don't want them.
http://www.ukscenario.com/forum/viewtopic.php?f=6&t=5248&hilit=qloader