Looking for an accessible app to make ringtones

Hi,
Do you know the way of preparing ringtones for my iPhone without iTunes and computer, only within an app? Can you recomend an accessible app that can make a ringtone from any recording stored either in the device or on external drives e.g. dropbox? I am aware of Apple limitations but look forward for your suggestions. I don't mind purchasing ringtones, they have reasonable prices, however, I can't purchase all tones e.g. these of my family or from my own recordings. Thank you for your replies.