I'd try to get in contact with the iTunes store for help. Every link that I find through phobos.apple.com just takes me to the front page of the iTunes store, so it's clear that the update is either gone or has been moved. Maybe they figure that everyone with interest would have bought it by now, especially since new devices ship with it.
If you can wait until early July I imagine the new update will include the JSU. If they've taken it down I can only imagine that they'll include it in this new one. Apple aren't going to exclude those who are willing to pay for the update.