Great thread up on Quora on the topic right now. Some of the top replies include:
Don't settle for local talent. Allow your sourcers to work virtually and scour the earth for the absolute best Sourcers you can find.
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Hire people who love the hunt. Full cycle or full desk recruiters often don't have the skills or interest to just be a Sourcer.
(via James Colino)
Hire through word of mouth. Sourcers (at least the good ones) pretty much know each other. Once you get a top performer allow them to pull together people they have worked with in the past.
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Make absolutely sure that your organization is ready and understands that sourcers are NOT junior recruiters. Professional sourcers are the equivalent to the Seals in the Navy. We are the best tactical recruiters.
(via Jonah Manning)
Full Quora thread here: What are the best strategies for building a Recruiting Sourcing team?