The Collaborative International Dictionary of English v.0.48:

Truster \Trust"er\, n.
   1. One who trusts, or credits.
      [1913 Webster]

   2. (Scots Law) One who makes a trust; -- the correlative of
      trustee.
      [1913 Webster]

WordNet (r) 3.0 (2006):

truster
    n 1: a supporter who accepts something as true [syn: believer,
         truster]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "truster":
   Christian, God-fearing man, accepter, believer, catechumen,
   churchgoer, churchite, churchman, communicant, convert,
   daily communicant, devotee, devotionalist, disciple, fanatic,
   fideist, follower, good Christian, neophyte, pietist, proselyte,
   receiver, religionist, religious believer, saint, the assured,
   the believing, the faithful, theist, true believer, votary,
   zealot



Bouvier's Law Dictionary, Revised 6th Ed (1856):

TRUSTER. He who creates a trust. A convenient term used in the laws of
Scotland. 1 Bell's Com. 321, 6th ed.