iPhone ringtones are purchased using the iTunes software, but you have to get iTunes to show you which of the songs on iTunes are available as ringtones.
Here’s how to set up iTunes to show you Ringtones:
1. Start the iTunes softare on your PC/MAC.
2. Select "Purchased" in the left hand menu (found under the ‘Store’ heading).
Okay, now you have a list of songs you’ve bought. By default it shows the name, Artist, Album, etc.
3. Right click any of the headings and you’ll get a pop-up menu. Select ringtones from the menu.
Now you’ll notice that you have a new heading that shows a bell icon. If any of the songs you have purchased have a bell icon next to them, the song is available as a ringtones.
As you browse through the itunes store, the bell icon will indicate if a song is available as a ringtones, but only in the list view.
USA users can find a list of the Top 10 iPhone Ringtones on the itunes Store main page.
UK users are basically f**ked, because NO SONGS in iTunes UK are currently available as ringtones. See Still no news on iPhone Ringtones for UK users.