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Different iTunes sections have differing amounts of free content. The Music store no longer has free songs available unless there is a special promotion, while the TV section still offers free episodes. The App Store and the Book Store have the most free content. Select the content type from the top of the iTunes window. You can click the "..." button to see additional types like Books and Apps. Once you've selected a type of content, click the "iTunes Store" tab. You'll find this on the right side of the iTunes window after opening the store. Depending on what Store you are viewing, it could say "Free Books" or "Free TV Episodes". If you're using the App Store, click the "Top Free Apps" link to browse through the most popular free apps available. In the iTunes U store, most of the content is free, so there is no "Free" section. Many podcasts are free as well, so there is also no "Free" section for them. There is no longer free music available in the Music store. Different sections will have different amounts of items. There are usually a couple dozen TV episodes available, and hundreds of Books. The "Top Free Apps" section of the App Store will display the top 200 free apps, though there are many more free apps than this. You'll see the "Get" button next to any item that you can download for free. When looking at a TV episode, you may have to scroll through the season to find the episode that is free. Clicking "Get" will begin downloading that content to your computer, which you can then use or transfer to your iOS device.
Know what you'll be able to find. Open the Store page for the content you want to find. Find the "Quick Links" section of the Store's main page. Look for the "Free Content" link. Browse and through the free items available. Click "Get" for any item you want to add to your library.