Follow these 4 steps below to download google play store app:
Check Current Version
Check the currently working version of your phone's Google Play Store app.
Download Google Play Store Using Apk
The non-default version (with respect to your mobile model) of Google play store can only be downloaded in APK file.
Enable Unknown Sources & Install
Enabling unknown sources option allows you to install application other then Google play store, which in this case is necessary.
Disable Unknown Sources
After installing the Google Play Store app, you are done and now you can again disable unknown sources option for security.