I had lunch a few weeks ago with a very savvy senior manager. She gave me a great sourcing tip. She said, “I look for candidates where other people seem to find candidates like the ones I'm looking for.” This is the same idea as loose networking.
As you wander through LinkedIn, looking for candidates, you'll find other companies looking for people just like the people you are looking for. Don't ignore those job postings. Look to see where else those companies have posted their jobs. Which job boards have they posted on? Maybe they know something you don't.
Consider trying those boards. Not all candidates go to Monster or Dice. Some go to Indeed. Some go to Craigslist. Some go to Simplyhired. And, remember there will always be a new job board every so often, sometimes every month! I'm talking to people now about a new job board, one that I will post articles on.
Remember, these companies are not always your competitors. But they are looking for people who are looking for people similar to your open req. You can take advantage of their legwork.
I had an argument with myself about whether I should even try to put a list of job boards in Hiring Geeks That Fit, because by the time I go to print, the list will be out of date. Even for an ebook, I would have to republish too often. I have decided against it. Should I have a link to a list of job boards that I can attempt to maintain? Would that be worthwhile? Let me know.