The Collaborative International Dictionary of English v.0.48:

Warrantor \War"rant*or\, n. (Law)
   One who warrants.
   [1913 Webster]

WordNet (r) 3.0 (2006):

warrantor
    n 1: one who provides a warrant or guarantee to another [syn:
         guarantor, surety, warrantor, warranter]

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

WARRANTOR. One who makes a warranty. Touchst, 181.