Think about the type of lyrics you're going for, and search for your writer based on the type of artists for whom he's written in the past. If you're looking for whimsical rhymes, perhaps find someone who writes lyrics in the vein of Eminem or Kanye West. If your track is more menacing or gangster in tone, go with someone known for a more gritty, underground style similar to that of Ghostface Killah or 50 Cent.
Develop an understanding between the artist and the writer. If the writer is looking to get paid, she'll agree not to receive any other credit than monetary for her work. If she's looking to get "put on" or become an artist herself, you may run into trouble.
Check trade magazines such as "Backstage" and the ads in local city papers, or even online classifieds such as Craigslist. People in the music business are always looking for work and often advertise their services in such publications.
Look for ghostwriters through services that specialize in hip-hop ghostwriting. Examples include online music sites such as MusicCouch and more professional services such as RapRebirth.
Scan record label publishing catalogs; companies such as EMI and Sony may have the names of writers who've worked on projects or have listings with agents or publishing companies that are always looking for new clients. Call the company if you need additional information.