About fourteen years ago, the FAA became aware of a gradual loss of generational experience in all walks of aviation, from design through manufacture to maintenance and operations. They embarked on a project to capture as much of the lessons learned over the years as possible. The website linked above is the outcome of that work. It is invaluable. To my knowledge, there are no other historical sources of accident reports that have any kind of organization paralleling this. Although the DOT Online Special Collections and Embry Riddle's Hunt Library do have extensive collections of accident reports, nothing approaches this organization.
Each accident page has a submenu to the left which connects to a very comprehensive library of work specific to that accident, from accident findings to precursors to safety assumptions right on through subsequent regulatory changes. In many cases, digital links to the actual accident reports are available at the bottom of the Accident Board Findings page.