It's not really that hard, if you have something like Bittorrent or something similar to it you can try looking for torrents for the entire soundtrack on a search engine of your choice (I did mine on thepiratebay.org, which isn't really a search engine but that's how I got it; probably answered your question already). Another option is using rar. but I don't know how to make good use of it.
Now if you only want the Promise, you can dowload the torrent still and check off everything else and recheck any songs you want. Just download it and you're done.
BTW, do check on seeds (or seed yourself after you finish). It helps.
If you're talking about "finding" it though, it's the second track on the first disc.