Very few. I have found Muzzleloader magazine on a newstand many years ago, but not for at least the past 15 years. I see Backwoodsman magazine in some of the better bookstore news/magazine racks, but you won't find Muzzle Blasts, or anything else there.
I lobbied the NRA for years to put its magazine on the news stands so that the non-shooting, but NOT anti-gun, public could have a first hand source of information about the NRA's position on various issues, instead of getting the distortions from the anti-gun media, and they finally have tried it out. I don't see it everywhere, but I have seen the American Rifleman, and another NRA publication on news stands.
No, they didn't give me any credit for my advice.