What city are you in? It really depends on your location. For example, I'm in London and when I started contracting I simply uploaded my CV to a job site and my phone exploded with calls from recruiters because the demand for contractors here is so high.
Which site lead to most high-quality leads? The most common freelancing sites just seem like a race to the bottom, competing only on price with developers in developing countries (heh)
For me it's http://www.jobsite.co.uk. The contracts are onsite, normally 3-6 months, sometimes rolling. Devs are popular with digital media agencies because the projects are short term. However, I don't know how in-demand devs are. Sysadmins/devops with automation and cloud experience are though.
If you use a recruiter then under UK law you have to sign up with an umbrella agency or run your own limited company as they aren't allowed to work with sole proprietors.
http://www.jobsite.co.uk/ Are you focusing on dev or ops? Don't really know how in-demand devs are right now but sysadmin/devops guys (esp with cloud and automation experience) are. If you want to go the contracting route then the contracts are on-site so you aren't competing with people in India, etc. Also, if you go through a recruiter (highly recommended) then by law you'll have to join an umbrella company or start your own limited company (not a sole proprietary)
