This works great for my Samsung SCH-A950 phone, but I think it would work for lots of others as well. You can take any mp3 file, and turn it into a ringtone.
You just need the following programs installed...
-iTunes (you should have that anyway) download
-Qualcomm PureVoice 3.1 for Windows download
Change the setting to wav 8-bit, Mono.
Then you can use itunes to convert the mp3 to an 8-bit, Mono, wav file by right clicking it.
The Qualcomm software is a little harder to use. Follow their instructions to convert the wav file to a QCP file. This QCP file is a valid ringtone format for many phones. Just move it to the phone somehow (I use my TransFlash memory card) and select it as your ringtone for one contact or for all.
Don't pay for ringtones when you can make them yourself. Comment if you have any questions.