It has been extracted from the Android 4.2 Jelly Bean platform
Android 4.2 Jelly Bean will bring along a nice range of enhancements, including a series of updated system applications, such as the launcher or the camera, and will also provide users with a new flavor of the Google Play app on their devices.Courtesy of a recently leaked Nexus 4 system dump, the Google Play app for the platform has been extracted and made available for download as a separate apk file.
Those who would like to give it a try should head over to this thread on the XDA-Developers forum to download it, copy it to the phone, navigate to it using a file explorer and install it.
They should also set the device to accept installations from unknown sources before proceeding with the process. On the said thread, you will also find info on other Android 4.2 apps available today, such as Play music, Voice Search for Play Store, Google Earth and more.