Well, I can't speak from experience but my guess is that most people in this part of the forum create the music themselves. Regarding your question, I guess that it differs quite a lot but a large part is contacts. Have a friend who knows somebody that needs audio? that works. Or being active on a forum like this or any other game development forum or advertising yourself there. Perhaps being active on soundcloud helps and just posting your stuff on facebook every now and then, just so people at least know that you're doing stuff with audio. A great thing would be to work on gamejams with people in order to showcase your ability to write music and fill up your portfolio.
And still then you might be finding your job by having a conversation with someone at a bus stop. You never know.