Download Android Marshmallow launcher:
Google announced next version name of Android and latest and the last Preview 3 of Android 6.0 built. So some awesome people fetched apk of the new stock Google Now launcher and it's no different than last preview available just a little bit tweaks and some improvements. The style of app drawer is still vertical on Nexus devices but not yet confirmed on other devices.
The apk will be supported by Android Version 4.1+ (Android Jelly Beans and above)
You can download apk from the link on the bottom: