I start from finding the title in YouTube 'best epic songs in 20??' (Not the title, there are always new songs as the years go by.) then I listen to every song that it plays and when I like it, I write the title down and who did it. and look for the original in YouTube. When I do, I record it into my iPad or memory stick.
Other ways I just randomly click links on the right in YouTube or listen out for epic songs in real life in movies or trailers and type down '(something something) trailer soundtrack' and if it does not show, I wait for a couple of weeks or months until the soundtrack gets released.
That is how I find my songs.