The Collaborative International Dictionary of English v.0.48:

Indorser \In*dors"er\, Indorsor \In*dors"or\, n.
   The person who indorses. [Written also endorser.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Endorser \En*dors"er\, n.
   Same as Indorser.
   [1913 Webster]

WordNet (r) 3.0 (2006):

endorser
    n 1: someone who expresses strong approval [syn: subscriber,
         endorser, indorser, ratifier]
    2: a person who transfers his ownership interest in something by
       signing a check or negotiable security [syn: endorser,
       indorser]

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "endorser":
   Maecenas, abettor, admirer, advocate, aficionado, angel, apologist,
   approver, backer, buff, certifier, champion, confirmer,
   cosignatory, cosigner, defender, dependence, encourager, exponent,
   fan, favorer, friend at court, guarantor, insurer, lover, mainstay,
   maintainer, notary, notary public, paranymph, partisan, party,
   patron, promoter, protagonist, ratifier, reliance, second,
   seconder, sectary, sider, signatory, sponsor, stalwart, standby,
   subscriber, support, supporter, sustainer, sympathizer,
   underwriter, upholder, votary, well-wisher