or for an m203 style set up you can just find a 37 launcher on a forum like ar15 or weapons forums. normally at least one on goes for under 150 bucks...can launch chemicals smoke flashbangs flares...check legality of these and any custom launcher. some of these custom plans they sell end up making you a felon

so before you add some huge launcher set up to your paintball gun check your local and state laws because you might have to pay a tax or have a stamp/license for it.