Q: How to purchase Shake Source Code?

Apple, to the best of my knowledge, discontinued all sales of Shake quite some time ago (close to two years, I believe), whether of the application or the source code. So unless someone else knows of a source at Apple you can still talk to, I think you're long since past the deadline for obtaining a license.

Regards

I think it would be best to contact Apple directly,
and I don't mean walking into an Apple Store.

Maybe you can start here:
http://developer.apple.com/appleapplications/shakesdk.html
and get on the list.