The Classics of the Old West series, like the other 'classics of' series, is a set of reprints. The originals in this case are books about the old west written by people who actually experienced it. Some of the authors of this series are well known, like Theodore Roosevelt, Mark Twain, Buffalo Bill and Washington Irving. I also find it odd that there is a biography of Billy the Kid written by Pat Garrett; since when do you get to write biographies about people you killed?
You see entire sets of these for sale on e-Bay pretty often. They are pretty expensive, though; usually going for over $125. Individual volumes also show up frequently on e-Bay. You can buy these volumes on Barnes & Noble or Alibris used, too, so that might be the best bet overall.