Google's official launcher was finally released on the Play Store a couple of months ago to much fanfare. Despite being your favourite home screen replacement, it's still not officially available for many devices. Here's how to work around that pesky limitation.
Fortunately, Google has actually made it pretty easy. Most of the code for the launcher itself is actually included in the Google Search app. However, the Search app does not identify as a launcher, which means it's not called when you press the Home button. For that, you'll need what's colloquially referred to as the Google Home "stub". This APK runs when you call the launcher by pressing Home, and then refers to the code in the Search app.
The Google Now launcher is designed to run on Android 4.4, but if you're running 4.1 and up, you may be able to get it working with minimal (if any) bugs. Head to this XDA thread and download the zip file. There are three APKs in the archive, but the only one you need to care about is GoogleHome.apk. Install this APK and press the Home button. Here you can select the Google Now launcher as the default, though it would be wise to ensure that the launcher works properly on your device first.
This method is unofficial and there's no guarantee that it will work properly on your device. Anything below Android 4.1 will not work as that's the lowest Android version that supports Google Now. However, that doesn't guarantee support above it. Your mileage will always vary. Once you've installed the stub APK, updates to the Search app can and will change how the launcher works because, as mentioned previously, most of the code for the launcher is contained within Google Search.