1. Determine your budget based on the cost of acquisition of a customer
2. Determine your venue (i.e. web based copy for a landing page; direct response mailing; email autoresponder series; etc)
3. Determine your desired outcome (i.e. metrics you will use to monitor your ROI)
Then either ask for references in your network or try posting an ad here: http://www.directresponsejobs.com/

The more specific you are about what you are looking for - the better your chances of finding it. Hiring a copywriter is just like hiring for any other position.

And remember the wise advice of "Hire slow, fire quickly."

Best of luck!