Have you tracked down as many of the descendants of your great-great-grandparents as possible? It may be that your cousins have family papers, mementos, letters, etc. that could assist you in your research. These items may have passed down to other branches of the family besides yours. Even individuals who are not interested every minute detail of the family tree may have information that could help you in your search.
Your great-great-grandparents could have over 1,000 living descendants as one set of mine do.
Your great-great-grandparents could have fewer than thirty descendants as another set of mine do.
But you will never know if you do not look.