You Mac users probably know about this already, but in case you didn't, go here: http://www.macheist.com/ There's a bunch of software that they normally charge for, only it's freeeeeee for the next few days. No idea what's there - I'm not a Mac guy - but hey, maybe you are. Enjoy!
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I don't know of one that will use multiple sources. You could see if there is a jigdo version of the ISO. This is something that Debian used to use for distribution. I believe that works the way that you want it to.