One of the main problems with Open Education Resources is that they are scattered all over the internet however the OASIS collection covers a wide range of sources in a very friendly format and so is a great place to begin your search. OASIS currently searches open content from 117 different sources including BC Campus, DOAB, Boundless, Oapen, Project Gutenberg, Open Stax, Open SUNY textbooks, Open Textbook Library, Project Muse, TED talks, etc.
Remember not all Creative Commons Licenses will allow you to do as you please with their resource. Make sure you know what license your resource has been given and what it means for its use. See under the Copyright and Licensing tab for more information.