I've looked around a little and haven't found any bang sticks - seems they're no longer "in" -
I suspect that if "firearms" are banned for taking gators, then bangsticks (which are certainly firearms) would be banned too. If you're sure they're legal, you can easily build one in any machine shop from readily available materials. A 12 ga shotgun shell fits 3/4" pipe perfectly... Many, many years ago, before I'd ever heard the term "bang stick" (and when we were ignorant of the ecological realities of shark behavior and populations) we built 'em ourselves.
*WARNING* These things are deadly in ways that make high powered rifles look like pea shooters. If you do manufacture one, make absolutely sure you include a safety in your design - with a bang stick the bullet or shot is incidental to the shock wave and the expanding gasses of the "bang" (which is surprisingly quiet, by the way) - an animal hit with a bang stick - especially a 12 ga - in a place that would be a mildly irritating flesh wound were it hit with a rifle - will be dead. Period. Humans too.
*BE CAREFUL*
Rick