If you find an OER (or multiple OERs) that you like, but it isn't perfect, consider adding your own edits. (Visit the "Licensing" tab or ask your library liaison if you're unsure whether the license allows editing.) Or, if you don't find anything you like, consider writing your own OER. The resources below will help you get started.
The Rebus Community is dedicated to supporting collaborative open textbook creation and editing. They have a forum dedicated to answering your questions, a list of projects in progress that you can contribute to, and a form to help you find collaborators. They have also published a number of helpful guides:
BC Campus is a project of the province of British Columbia. In addition to having an open textbook repository, they have also published several guides to editing and authoring open textbooks: