It's a labor of love and takes lots of investigation and research on your part.
All of the ideas mentioned above and many, many more. The best thing is to start somewhere, anywhere, and branch out from there. Eventually, you'll find the places that suit your tastes and find your own niches.
For me, I've made lots of friends, including some of the musicians and label owners themselves, and found other resources over time which then snowball into others. Eventually you will stumble into some of the right places. By 'right', I mean right for your tastes, not necessarily everyone elses.
One thing that I suggest is that you play around with a variety of websites, magazines, radio stations, message boards, etc, and not limit yourself to only a small few. But again, this takes a labor of love only for those who are truly serious if your goal is to stay ahead of the game. Even then, don't be discouraged when you see that you've still missed out on not being the first in some group to find any said band. No one reins supreme in that regard.
