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]