Right click on the song that you wish to edit (ctrl+click if you're using a one-button mouse). An option appears which states "Show In Finder." Select that.
Now you will see the file in Finder.
You've got lots of music in your iTunes, much of it you believe that would work well as parts in newer works that you'd like to create. Remember to categorize it by BPM using the method described in the techniques blog [here].
This article will show you how to bring iTunes into Reason software, using Recycle. Once you learn it, this will change your music collection forever and grant you honorary SpacePirate status.
Amadeus Pro ($60) (or any other basic audio editing program, for that matter), you will find that the file is right in front of you, to copy and edit as you would a bit of text in a word processing editor.
The trick is to find the parts that will work the best with remixing. The parts that work best are sections of the song that have stripped-down parts, containing either no vocals, or only instruments, such as an introduction part that just contains the beat.