Using OER Commons (https://oercommons.org)
One commonly used starting point is OER Commons. Created by the Institute of Knowledge Management in Education, OER Commons has partnered with over 120 major content partners to provide a single point of access through which educators can search across collections to access over 30,000 items, find and provide descriptive information about each resource, and retrieve the ones they need. These resources are publicly available for all to use principally through Creative Commons licensing.
A basic search will allow you to look for content in a variety of subject areas and grade levels. From there, you can further narrow your search by the TYPE of content that you are looking for (ranging from full courses to textbooks to lecture notes to activities).
When the results are displayed, you can select a source and view/download the OER content directly. It will also give you a preview of the source along with an abstract, information on where it resides and information on how popular the site is. You can also see various tags to link to other similar content and the Creative Commons license that is used (to help you determine conditions of use). (See January 2017 TLETC blog for a review of Creative Commons licenses).