I was never tied to a specific contracting agency.
When I was contracting, I went through PDS Tech. That was one of the 10 that Lockheed used. PDS had the best deal for me. It may be different for IT but in software engineering, I just had my resume on Monster.com and contracting houses would call to tell me of an opening in some company bla bla bla. If they sound legit, I gave them a copy of my resume. If not, no thanks. Same story for all my buddies at work that were job shoppers.