: SoundBible and Soundsnap provide categorized alert sounds, including emergency signals like the "Tornado Siren II," often available in both MP3 and WAV formats.

: You can move your downloaded MP3 to the Notifications folder on your internal storage, then select it via Settings > Sound & vibration .

: Sites like Pixabay and Mixkit offer high-quality, free alert sound effects ranging from "Positive Notification" to "Sci-fi Intrusion".