Apple Music: This platform offers a vast library of songs, including Queen's discography.
Amazon Music: You can find "We Are the Champions" on Amazon Music. Depending on your subscription (Amazon Music Unlimited), you can download the song for offline listening.
Google Play Music / YouTube Music: These platforms also host the song.
iTunes: You can purchase and download "We Are the Champions" directly from iTunes.
While Apple pushes its streaming service, the iTunes Store still sells individual tracks. Their M4A files are easily convertible to MP3. The sound quality is pristine, featuring the original 1977 mastering.
If you're looking for an official audio or a specific performance, YouTube is a good source. Just ensure you're using the official channels (like Queen's official YouTube channel) to support the artists and rights holders.