Since the game was delisted by Rovio, the most trusted community sources for the APK are:

On newer versions of Android, the game might get stuck on a loading screen. You can often fix this by turning off your internet connection before opening the game, then turning it back on once you reach the main menu.

Generally considered the safest site for archived apps.

A reliable alternative that often keeps older versions of games.

Since servers are no longer officially supported, your old cloud saves might not sync. You will likely have to start a new adventure.