The Collaborative International Dictionary of English v.0.48:

Tipster \Tip"ster\, n. [Tip a hint + -ster.]
   One who makes a practice of giving or selling tips, or
   private hints or information, esp. for use in gambling upon
   the probable outcome of events, as horse races.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

tipster
    n 1: one who sells advice about gambling or speculation
         (especially at the racetrack) [syn: tipster, tout]

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "tipster":
   adventurer, adviser, announcer, annunciator, authority, betrayer,
   betting ring, bettor, boneshaker, canary, cardshark, cardsharp,
   cardsharper, channel, communicant, communicator,
   compulsive gambler, crap shooter, dopester, enlightener,
   expert witness, fink, gambler, gamester, gossipmonger, grapevine,
   hazarder, informant, information center, information medium,
   informer, interviewee, monitor, mouthpiece, nark, newsmonger,
   notifier, petty gambler, piker, player, plunger, press,
   public relations officer, publisher, punter, radio, reporter,
   sharp, sharper, sharpie, snitch, source, speculator, spokesman,
   sport, sporting man, sportsman, squealer, stool pigeon, talebearer,
   tattler, television, teller, tinhorn, tout, touter, venturer,
   wagerer, witness