When I saw that this documentary was streaming on Netflix, I got pretty excited! I've been slowly building a (small) collection of gig posters over the past 10 or so years, so any opportunity to learn about the history and origin behind this art form is welcomed by me! This documentary, titled Just Like Being There, had some great info and background on the American gig poster. Some of the best and most well known gig poster designers/illustrators were interviewed and had some interesting views about what a gig poster is, what it should be, and what it was. Do check it out.
If you're interested in starting a collection of your own, check out Lonestar Posters. It's where I went to get most of the posters I've collected. They have a really great selection.