If you have ever installed it before, it may still be in the apps folder of iTunes.
One suggestion if you have an older device is to use something like iMazing to back up the apps you use prior to any restore. Not a bad idea for any device. I just re-installed my iPhone 6 from scratch, and many apps are showing as no longer available on the app store at all. I have been able to restore them from backup and they still work fine.