The easiest way I could think of doing this was to download and install Oracle JDeveloper 12c on your desktop machine. JDeveloper is free to use, though it is quite large coming in at a ~2Gb download.
JDeveloper offers complete end-to-end development for Oracle's platform and Oracle's applications.
Once JDev is installed, orapki can be found in the following locations:
The reason I said 'sort of' earlier is because JDeveloper installs a copy of Fusion Middleware for you. It's not quite the same as the one you get when you install a production version but for development it serves the same purpose.
When downloading JDeveloper make sure to select the 'Studio' edition because the 'Java' edition doesn't include the middle tier and its tooling.