I hesitate to suggest it, but you might try the site elance.com. Clients post summaries of their jobs. You submit a quote and a deadline, alongside everyone else interested in the job. As you might expect, bidders with more experience and better credentials tend to get most of the jobs.
Be prepared to do a lot of searching. There are several categories of writing jobs, and the clients often assign the wrong categories to their jobs. Occasionally someone posts a ghostwriting job.
The reason I hesitate to suggest eLance is because I tried to supplement my income by doing writing work. After a long time I got one contract, and I had a poor experience with the client. You may have better luck.
I've made a few thousand dollars on Elance over the past couple of years, and have had mostly good experiences. Some of it was technical writing, some legal, and a couple of ghostwriting jobs. Just watch what you're bidding on and stay away from people who seem to be looking for work done at the lowest possible price.