2 places:

1. Premium job boards. AuthenticJobs used to carry much more freelance work, but basically I'd find places where people had paid to post a job, and I'd do my darndest to impress them with my opening email.

2. My professional network. A couple of old contacts referred me to excellent agencies. Those connections still pay dividends today.

Hope that helps. These comments are giving me ideas for "Everything I know, part 2".