Play Store

Free version
The easiest way to get hold of Data counter widget is to install it directly from Play Store.
Get it on Google Play
Pro version
You can also get the paid pro version from Play Store. Having that app unlocks extra features in the free version.
Get it on Google Play

Without Play Store

If you for some reason cannot use Play Store it's possible to install the free version of the app directly using the APK file. Every version is available from https://android.roysolberg.com/datacounter/releases/ as seen in the frame below. Generally a more recent version number is better. But if you are running an older Android version (1.5-5.1) you want to download version 2.3.2.

Changelog

If you want to know what has changed from version to version there's an extensive changelog of the app.