Pretty much everything from 1839, the year of the invention of Photography, until today has been photographed. Some of these images have become quite expensive depending on various factors. However, many photographs are still quite affordable. It is quite possible for someone to build a collection of original photographs for a reasonable amount of money especially if you are interested in a specific subject area.
If you collect photographs, then I believe that this is a site you will enjoy looking at. Photographs are available in many different subject areas such as: military,erotica, crime, sports, science, and aviation to name just a few.
Also, many processes and types of photographs are represented: daguerreotypes; ambrotypes; tintypes; cdvs cabinet cards, cyanotypes, salt prints, albumen prints, silver prints, snapshots, hand-colored, lantern slides, and autochromes. Not only are vernacular photographs available for sale, but images by well known photographers as well. I also hope that you will take some time to visit The Museum Of Vernacular Photography. As you browse through the site, please let me know if I can help you in any way.