I think most DJ's have a passion for music. We listen to so many requests and talk with so many other like minded people that the community is a continued source of music education. Professionals also use music pools and companies like TM Studios or Promo Only, but that mostly just gets us up on what is popular or will be popular.
The real question is how do the trend setters in the music industry find what the next big thing will be? Is it just a guess as to what will be most lucrative or is it a desire to find originality and musical progress?