For me, it has been a mixture of 1)personal relationships 2) doing market research, 3) Linked In, 4) Internships.com
For everything from investors, advisors, and team members, I first started with my own circle of friends. While doing market research on my industry, I found several blogs, and articles by others who then became team members. Linked In has provided me with the most members of my team, and Internships.com was great for an entry level type team member. They weren't getting anything at first, but proved so valuable, that they are now a team member.
Hope those points help out!