Theres a lot of agencies like 52ltd.com that help freelancers find advertising jobs and creative gigs. They specialize in copywriting jobs, graphic design, flash jobs and web design job placement. 52 Limited also works closely with Nike & Adidas who out sources work to them. If you'd like to work with either of those 2 giants, submitting your resume to 52 is a good place to start.
http://www.52ltd.com