I use a number of sources. I started out using Guru.com but the jobs were low-paying and unsatisfying. Then I moved on to optimizing my Web site and marketing my services. I also find a lot of leads through Deb Ng's Freelance Writing Jobs blog as well as jobs.problogger.net.
getafreelancer, guru and many other freelance web sites, but usually hard to compete there... you should try to search DP forums, there was a thread with many copywriter web sites listed that also included forums and job offers/requests....
My copywriting clients almost all come from direct referrals from other clients, friends, colleagues, etc. Assuming you actually mean copywriting when you way that (and not just content / article writing), if you're just starting out consider doing some free or inexpensive brochures, Web copy, etc. for local non-profits to help build up your portfolio first. If you start by advertising on forums and freelance sites, you'll be facing an uphill battle for a long time to come as far as earning any serious money goes.
I'm interested in this thread and tried to search, I believe this one is the thread (just sharing it if someone want to see).