M-80s are prohibited by federal law. That's not to say that you can't find them, but they're not produced by any of the commercial fireworks companies.
If it says 'Blackcat' or 'Bluemoon' or 'Thundercat' or the name of another fireworks company stamped or printed on it, its not an M-80. Those are "legal" M-80s, which are about half the power of a real M-80.