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audiko.com so far is the best, it allows you to make your own and download it in iPhone ringtone format as well as search through there database completely for free.
 
If you like adding cool effects or producing your own ringtones with a synth and a MIDI hook up though, you could always download Audacity and the codec to save it as a M4R file. I remix songs with Audacity and auto tune my crappy voice. But thats if your doing the above.
 
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