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Ringtones and alert tones cannot be purchased from your computer, but can be purchased using your iOS device with iOS 5 or later. Tones can be purchased on iPad, iPhone (3GS or later), and iPod touch (4th generation or later).
To locate tones in iTunes on an iOS device:
- Tap the iTunes app.
- Tap Tones. If Tones is not visible, tap More > Tones.
- Browse for tones. Once you purchase a tone a dialog is displayed. The dialog gives you an option to set your new tone as your default tone or assign it to a Contact. If you don't want to do anything with the tone just yet, simply tap Done.
- Tones purchased using your iOS device are synced to your iTunes library when you connect your device to your computer. On an iPod touch or iPhone, tones can be found in Settings > Sounds. On an iPad, Tones can be found in Settings > General > Sounds.
http://support.apple.com/kb/HT1491