Ah, it seems the only place to get it is on F-droid, the open source app "store".

The Endless Sky game is made for PC, but there's a fork that maintains compatibility with android. If you have F-droid, search for it there. If not, you can download the APK file from the F-droid page for Endless Sky.

If this is your first time installing something that isn't on Google Play, then your phone is going to fight back. But it generally should tell you what you need to do before you can install apps from outside the Play app store.