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Generally you'd have to pay for the ringtones you want to create using the iTunes Music Store.

However, there are other alternatives. If you're using a Mac just download the latest version of iToner, it's as easy as dragging mp3 or wav files into the application, it then syncs with your iPhone.

If you're using Windows I really don't know if there is an application that does the same as iToner but a Google Search would probably clear that up. :)
 
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