An 870 Wingmaster seems to be by far the most popular, I myself like my Ithaca 37 Featherlight. If you want an autoloader, look at a Benelli or Beretta (I shoot a Beretta 391 myself), for doubles look at Berettas, Brownings, and some of the others from name brand manufacturers. I'd probably shy away from some of the cheapy imports though.
12 guage is always available at any store that remotely might sell some shotgun shells.
