Run by Sony Music, is another excellent Japanese source for legal Gundam Seed OST download files. It offers both MP3 (320kbps) and FLAC. The interface requires basic Japanese reading skills (or a browser translator), but the catalogs are complete, including the rare Gundam Seed Destiny OSTs.
For legal and high-quality listening, you can find the Gundam SEED discography on major streaming platforms. These services typically offer a "Download" feature for offline listening with a subscription: gundam seed ost download