You can download it for free from the Google Play Store. However, to get the Pixel 2 Style Search bar, you have to upgrade to Action Launcher Plus (Paid).
Once you've got the Action Launcher Plus with v29.0 update, go to Settings -> Quickbar -> Style. From here, you can select "Search box (dock)" to get the Pixel 2-style search bar right on your home screen.
You can also adjust its contours, opacity, position, and other aspects. This App is available for all devices with Android Lollipop or higher version.
Download it from Play Store