A flare gun is a flare gun, not a firearm. Although, if you tried robbing a bank with it someone might change the definition. If used as a means of force to commit a crime it could be construed to be a weapon, not a firearm...but so can a baseball bat, flashlight, hammer or breaker bar.

They are sold over the counter at marine supply stores, Wally World, Cabela's and various other sporting goods stores. There is no 14 day waiting period or pistol permit requirement to purchase one.