Secondary sources are published materials that examine or describe something else, usually something original. Those original materials cover a wide variety of objects, including:
Secondary sources are published in a variety of formats. This is what you need to look for:
So how do you find secondary sources? Using library resources, that's how!
People have been publishing for mass consumption on paper since 1440 with the invention of the printing press. That means there's a LOT of resources at your disposal. How do you sift through it all? Practice these tips to narrow down the number of results you get when researching.