There are a number of ways to find primary sources. It takes some practice and trial & error. Once you find some good subject headings you can use them in multiple catalogs, the WU Catalog, HathiTrust, WorldCat.
Start with keyword terms and see if the subject headings associated with a work lead you to a primary source
the appendices of books you check out often contain documents, letters, reports, images
the articles you read will reference primary sources, which you will then have to try and locate
many national libraries list digitized materials from their websites
specific (form) subject headings indicate primary source materials
Early Arabic Printed Books from the British Library is the first full-text searchable digital library of early printed books in Arabic script. Covering religious literature, law, science, mathematics, astrology, alchemy, medicine, geography, travel, history, chronicles, and literature, and including European translations of Arabic works and Arabic translations of European books, it exemplifies the long exchange of ideas and learning between Europe and the Arabic-speaking world.