One of the challenges is to decide which source(s) to choose from the myriads of choices...
Best to start searching in an OER repository. OER collections like MERLOT and OER Commons are resources created by the OER community. You can also contribute to these repositories through creating, licensing and subsequently depositing your OER in one of these collections.
The lists of resources under the Best Bets sub-tabs are by no means comprehensive but they can be used as a starting point for work in any discipline. Note that not all of the learning materials in these repositories and sources are OER for modifying. However, most of the content is freely available under Fair Use and/or with attribution.
Use the Advanced Search feature when available. You may limit your search by type, educational level, year...
Start with broad terms (e.g. disease instead of cancer) and then narrow.
As you narrow, think about disciplinary language. Is there something more specific or technical that might best describe your topic?
If you still are not getting good results, try to start with the browsing feature (even if it's very broad). Sometimes the term you are searching is not used but you still know it would be under a broad subject like "economics" or "communications".