Not sure that there really is a 'legal' way to download music without paying for it. Unless you go to bandcamp and an artist has a pay what you want that starts at $0. http://bythebirch.bandcamp.com/releases
Debussy's Clair de Lune (or "Moonlight") is a beautifully expressive piece of music, with a dreamy quality making it ideally suited for wedding and funeral services, whether held in a church or elsewhere. Because of its mix of long sustained and shorter notes, it is best played on a piano but experience organists can play it with care. Download the piano sheet music, midi and mp3 files.