You mean like this one? MP3Cutter 4 - The One And Only FreeWare MP3 Cutter

It will copy and then remake the header the section of an mp3 file of your choosing losing zero quality. It's great for when you want part of a music track as a ringtone.

note: XP only I think, but you didn't specify operating system.

Otherwise the best or rather easiest way is just buy them, but to me it feels like a ripoff when you can rather just rip a CD or download/buy an entire track using the same amount of money and just use above program.