The best reference for finding staff to work your party is by word of mouth. However, there are many party staff or even temporary staffing agencies with excellent reputations (and more importantly workman's comp insurance) that you can research and feel confident about using for your event without the sign-off from a person that you know.
If you've hired a caterer that has an in-house staff or some one they use all the time, I'd go with them!
If your finding staff on your own, ask your friends, family, and work buddies for serving staff recommendations for your party from bartenders to waiters.
Check on line or in your local phone book under "Party Staff, Party or Event Planning, Temporary Staff."
Call a local restaurant, which you frequent and trust, and ask if someone would be willing to work a private party.
After you've spoken to a few folks and feel comfortable with your initial picks, set up an appointment to meet them, if appropriate, or the agency contact in person. Or, proceed to Tip 2: Questions to Ask When Interviewing Serving Staff or a Staffing Agency.