WordNet (r) 3.0 (2006):

narc
    n 1: a lawman concerned with narcotics violations [syn: narc,
         nark, narcotics agent]

Moby Thesaurus II by Grady Ward, 1.0:

103 Moby Thesaurus words for "narc":
   Bow Street runner, FBI, FBI agent, Federal, G-man, MP,
   Secret Service, Sherlock Holmes, T-man, bailiff, beadle, beagle,
   bear witness against, betray, betrayer, blab, blabber, blabberer,
   blabbermouth, blow the whistle, bound bailiff, captain, catchpole,
   chief of police, commissioner, constable, delator, deputy,
   deputy sheriff, detective, fed, federal, fink, flic, gendarme,
   government man, hotel detective, house detective, house dick,
   inform against, inform on, informer, inquiry agent, inspector,
   investigator, lictor, lieutenant, mace-bearer, marshal,
   mounted policeman, officer, operative, patrolman, peace officer,
   peach, peacher, plainclothesman, police captain,
   police commissioner, police constable, police detective,
   police inspector, police matron, police officer, police sergeant,
   policeman, policewoman, portreeve, private detective,
   private investigator, rat, reeve, revenuer, roundsman, sell out,
   sergeant, sergeant at arms, sheriff, sleuth, snitch, snitch on,
   snitcher, spy, squeal, squealer, stool, stool pigeon, stoolie,
   store detective, superintendent, talebearer, tattle, tattler,
   tattletale, tell on, telltale, testify against, tipstaff,
   tipstaves, treasury agent, trooper, turn informer,
   whistle-blower