Depending on what you're looking to do, you'd probably be better off without one. Of the dozens of trainers I've seen come and go from my gym maybe 2 have a decent knowledge of how to build muscle.
Most trainers are taught (if they've been taught at all) to use low wieght/high reps to avoid injury risk and being sued but those programs don't know much of anything about getting stronger or leaner.
The trainers at the Gold's Gym I go to in downtown LA are extremely knowledgable, professional, and very qualified. I however, being the mountain of magnificence that I am, do not require their aid.
Train heavy, intensly, and with strict form. Give yourself lots of rest and remember that your diet is more important than rest or training. You grow after you workout and this is where your diet comes in.
I second finding the biggest baddest guy (or woman) around and asking them for advise. They obviously know what works (so long as it's steroid free).