We have had good experiences with Elance.

Here area a few tips:

-You can post a project but also explain that you will pick 2 (or 3) service providers. This way you can let them compete against eachother. You will have to pay a little more the first time but you can be sure that you will end up with a good job and you can choose that person in the future.
-You can also split the job in parts. And let a few service providers compete against each other for the first part. You can then take the best of them for the remaining parts.

Eric