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]