You may choose to create or modify your own Open Educational Resources if you do not find resources available that suit your needs. For an excellent overview on creating and modifying OER, take a look at Affordable Learning Georgia's guide to Customizing and Authoring Content and the Affordable Learning Georgia's tutorial for Creating and Modifying Open Educational Resources.
Creating an OER is similar to authoring any other document, except that you are assigning that content an open license which usually allows for its free use and re-purposing by others.
Things to consider:
Modifying an existing OER is a a simpler way of adopting content to suit your needs than creating a new OER. Before modifying an OER, check that the Creative Commons license does not contain a "no derivatives" clause. If it does, you do not have permission to modify the work. Search for formats that are conducive to modification, such as .docx and .rtf. Once you've downloaded and modified the OER, you can upload it to many of the sites listed in the "Sites for Creating and Hosting OER" section (See below).
It can be overwhelming to create and/or modify an OER for your classes. In addition to the wealth of information available from places like the ASCCC OERI and other OER content creators, there is an excellent OER text about publishing OER textbooks. The Rebus Guide to Publishing Open Textbooks (So Far) by Apurva Ashok and Zoe Wade Hyde is an evolving textbooks that takes you through considerations you may not have thought of such as determining your project scope, putting together a team to work on an OER, and maintaining the resource past the initial release date.
Sites to upload and share your materials:
Sites to create and share materials directly in an educational repository: