To get apk files of any app you need to have a app installed in your phone. The app name is called apps backup and restore. If you haven't downloaded yet you can download it by this link :- https://play.google.com/store/apps/details?id=com.touchfield.appbackuprestore
After installing Now open the app and select installed apps you want apk files for.
After selecting installed apps you have to touch the backup button available below in the app. After pressing the button your apps backup will be completed and you apk files will kept in the apps backup restore folder. Whenever you need to install them you can install them with help of backup and restore app anytime.
To restore them open the backup and restore app and select archive folder then select the apps you want to restore and press the button restore then it will installed by pressing install button in the app.
Comment below if you have any queries..