I made my own using a free piece of software called Audacity. I went my own way with it, but I think there's a guide on how to do it floating around somewhere - let me see if I can dig it up
EDIT: Here you go;
You can find apps for the itouch and iphone for about 99 cents that have both preprogrammed intervals and create your own intervals.
But if you just have an ipod, Audacity is the way to go.
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