Sites like Project Gutenberg or Internet Archive offer free downloads of historical texts regarding 18th-century piracy (the "Golden Age") and academic papers on secret societies.
Sites like Vecteezy or Freepik offer free skull-and-crossbones icons and illustrations for personal or commercial use. Which of these are you most interested in exploring? Skull and Bones Free Download
When looking for a it is important to clarify whether you are referring to the Ubisoft pirate action game or the historical/secret society topic. 1. The Video Game ( Skull and Bones by Ubisoft) Sites like Project Gutenberg or Internet Archive offer
If you are researching the secret society (Yale University) or general pirate history: Skull and Bones Free Download