I am a SEO content writer. I wish to make my career as freelance ad copywriter. How can i go about it?

I am a freelance content writer with an expertise of over four years in writing web content of all the types. I wish to start a career as a freelance ad copywriter. How can I go about it? Where can I get copywriting jobs for advertisements? I am speciallty interested in punchline/tagline writing. How can I get a freelance punchline or script writing job?