If you have a copy on your hard drive you will find it, by default, by following this path: youruserfolder/Library/iTunes/iPad Software Updates. (You access the Library folder in your user folder by holding down the Option key in the Finder and choosing Go > Library.)
The Mac will delete this files under certain circumstances so you may not have a copy of the latest one. If not, launch your favorite web browser and search for "download ipsw". You'll find a number of sites that offer links to the file you need. Just be sure to choose the correct one based on the device you're using.
When we discuss upgrading to iOS 8 in that future article I'll also provide steps for rolling your device back to iOS 7 (at least for as long as you're allowed).
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