The Collaborative International Dictionary of English v.0.48:

Sustainer \Sus*tain"er\, n.
   One who, or that which, sustains. --Waterland.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sustainer
    n 1: someone who upholds or maintains; "firm upholders of
         tradition"; "they are sustainers of the idea of democracy"
         [syn: upholder, maintainer, sustainer]

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "sustainer":
   Maecenas, abettor, admirer, advocate, aficionado, alpenstock,
   angel, apologist, arm, athletic supporter, back, backbone, backer,
   backing, bandeau, bearer, bra, brace, bracer, bracket, brassiere,
   broadcast, buff, buttress, cane, canned show, carrier, cervix,
   champion, commercial, commercial program, corset, crook, crutch,
   defender, dependence, electrical transcription, encourager,
   endorser, exponent, fan, favorer, foundation garment,
   friend at court, fulcrum, girdle, guy, guywire, jock, jockstrap,
   lover, mainstay, maintainer, mast, neck, network show, newscast,
   paranymph, partisan, patron, promoter, prop, protagonist,
   radio fare, radiobroadcast, rebroadcast, reinforce, reinforcement,
   reinforcer, reliance, rerun, rest, resting place, rigging, second,
   seconder, sectary, serial, shoulder, shroud, sider, soap opera,
   sound effects, spine, sponsor, sportscast, sprit, staff, stalwart,
   standby, standing rigging, stave, stay, stick, stiffener,
   strengthener, support, supporter, sympathizer, taped program,
   upholder, votary, walking stick, well-wisher