Swati, good afternoon. Let me preface my answer with a brief disclaimer: I am not a trainer, and I don't have all the answers. However, I am adept at tackling your question on a micro-scale with my firm's clients. While you may personally be looking for 10k SAP consultants in North America with specific module experience, I often find myself working on building and developing talent pools that are 100 to 200 candidates large in the entire U.S. In some cases, the pool is even smaller than that . . . and there is ZERO online presence.
From that angle, let me offer the following:
Truth: There is no silver bullet to your question. With some of our projects, there is no online mechanism that is going to consistently produce the results we're after because such a presence sometimes constitutes a threat to national security . . . and in many cases, there are no silver bullets that are going to help you telephonically penetrate a company with any degree of success. Think I'm lying? Try generating names on the 10 people at a Weapon Systems Manufacturer that are Top Secret cleared (with Polygraph) and have been working on nuclear-tip projects for the last 6 months (that haven't seen daylight as they've been literally underground). Think you can reach a guy that is in a biohazard protection suit . . . who can't even speak to the person next to him without a microphone . . . ? For these people, their families are even unaware of their actual location and movement . . . so to all, I say "Good luck."
In that sense, consider yourself lucky if you're only looking for some tough-to-find IT candidates (and that's an assumption on my part). What I can suggest is to shift your mind from looking at 'resumes' as bees in a beehive. Perhaps you need to create a beehive of your own.?. There isn't always a network or job board built for you already - you may have to create instead of leveraging what's already been created. Perhaps you need to make it easy for the "right" talent to find you instead of trying to make it easier for you to find them.?. What I'm suggesting is to shift your paradigm - think 6" instead of 40,000 feet.
Now, I don't want to romanticize this gesture, so if it's not organizationally possible for you to do this (due to your value proposition or client pressures for "right here/right now") . . . then I'd recommend contacting trainers and/or consultants.
For a shameless plug, the first thing you can do is contact me and my firm. We are experts in special operations strategic-sourcing projects. Are you after a target of opportunity and are having trouble crafting and executing a strategy? Are you having problems with the weapons, resources, and tools at your disposal to take the objective (i.e. the 'talent pool')? Then call us. We're not miracle workers, but we can do some cool things and stack the deck in your favor to win more than you lose.
Now, if that isn't feasible, I recommend that you call the trainers that can help you better execute on your end, and based on the way I'm seeing your question, you may need both telephone and internet sourcing training. If so, then:
Internet: Moises Lopez (Sourcing Corner) or Shally Steckerl (Job Machine, now merged with Arbita)