You may gather education resources from open access journals, books, videos etc.
Open education resources are searchable from e.g. web portals, with Google Advanced search or with Creative Commons Search.
"In the Library of Open Educational Resources (OER), you can search for, find, compile and save your own open educational resources from all levels of education. Gradually introduced from 2019 to 2020, the service and its OER are available for use by teachers, learners and everyone else in the country. Visit aoe.fi to find out more about the development of the service and keep up to date with its progress."
The series University of Oulu Study Materials publishes the print version of the book, the online version or both of them.
Opening your own study material is e.g. video of your lecture in YouTube or Vimeo or sharing slides in SlideShare.