On-line services like freelancer.com can be quite useful. I've had some bits of work that way myself. If you are just starting out, it's worth registering with them.
But, on the whole, I don't find them all that satisfactory. It's much better to deal with clients on a personal basis, and face to face if possible. From their point of view, the work you are doing is really important. You are promoting their products, helping them communicate with their customers, and enhancing their image in the media. You need a good relationship with the client based on mutual trust. I personally don't think you can get that from an on-line agency.
And the best way to find the clients? Well, it comes down to the traditional routes like referrals, repeat business, social contacts, and simply telling everyone you know that you are in business. Having a decent website is pretty essential as well.