Years ago, I would look up the celebrities parents and then track down their phone number. Then I could look at the parents phone bills and see who they called in Hollywood/New York/wherever, which almost always led to the celebrity. But this was back when normal people only had home phones and there weren't laws about getting phone tolls. Plus it's really hard to get tolls these days AND you have to figure out parents cell phone numbers instead of home phones, which is a little harder.
I know all that information is useless to you, but it may steer you in the right direction. What celebrity are you looking for?