If you have an mp3 file of what you wish to be a ringtone, could can put it on the phone and open it in the file manager or music app of your choice, press the ⋮ options button and there should be an option to set it as a ringtone.

... It beats the 2000's hell of buying a novelty ringtone for 99p and ending up in some small print recurring charge for a year.