You cannot buy lossless files on iTunes (AAC). Amazon Music hides them behind confusing tiers. And torrent sites? They are a wasteland of mislabeled "FLACs" that are actually just upscaled MP3s (the audio equivalent of putting a Ferrari badge on a Fiat).
FLAC is widely used in digital preservation and archives because it perfectly preserves the source material.
: A straightforward digital music store that provides high-quality FLAC downloads for many mainstream releases. Verification and Troubleshooting