The Collaborative International Dictionary of English v.0.48:

Backer \Back"er\ (b[a^]k"[~e]r), n.
   One who, or that which, backs; especially one who backs a
   person or thing in a contest.
   [1913 Webster]

WordNet (r) 3.0 (2006):

backer
    n 1: invests in a theatrical production [syn: angel, backer]

Moby Thesaurus II by Grady Ward, 1.0:

119 Moby Thesaurus words for "backer":
   Dionysus, Maecenas, Samaritan, Santa Claus, abettor, acquaintance,
   admirer, advocate, aficionado, aid, aider, ally, almoner,
   almsgiver, alter ego, angel, apologist, assignor, assister,
   awarder, befriender, benefactor, benefactress, benefiter,
   best friend, bestower, bettor, bosom friend, brother, buff,
   casual acquaintance, champion, cheerful giver, close acquaintance,
   close friend, conferrer, confidant, confidante, consignor,
   contributor, defender, dependence, donator, donor, encourager,
   endorser, exponent, fairy godmother, familiar, fan, favorer,
   fellow, fellow creature, fellowman, feoffor, financer, friend,
   friend at court, funder, giver, good Samaritan, good person,
   grantor, grubstaker, guarantor, help, helper, helping hand,
   imparter, inseparable friend, intimate, investor, jack-at-a-pinch,
   lady bountiful, lover, mainstay, maintainer, meal ticket,
   ministering angel, ministrant, neighbor, other self, paranymph,
   partisan, patron, patroness, philanthropist, pickup, presenter,
   promoter, protagonist, punter, reliance, repository, second,
   seconder, sectary, settler, sider, sponsor, staker, stalwart,
   standby, subscriber, succorer, sugar daddy, support, supporter,
   surety, sustainer, sympathizer, testate, testator, testatrix,
   underwriter, upholder, votary, vouchsafer, well-wisher