You can, really for free! Why pay when it is free?
Hint: Use freeware software like Audacity to open non-DRM and non -protected music files
then cut the part of the song you want
then export it as mp3
send it to your phone via email or bluetooth or cable hock-up
If the file is DRM or protected you still can do this put first you have to convert the file to a non-protected format
There are legal and illegal ways to do this
I will mention a legal way:
Purchase (or download the free trail version) of Sound Taxi.
*With the free version you can only convert the first minuet or two of each song, but for ringtones at the beginning of a song, this is fine!)