The Collaborative International Dictionary of English v.0.48:

Upholder \Up*hold"er\, n. [Up + holder. Cf. Upholsterer.]
   [1913 Webster]
   1. A broker or auctioneer; a tradesman. [Obs.]
      [1913 Webster]

   2. An undertaker, or provider for funerals. [Obs.]
      [1913 Webster]

            The upholder, rueful harbinger of death. --Gay.
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   3. An upholsterer. [Obs.]
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   4. One who, or that which, upholds; a supporter; a defender;
      a sustainer.
      [1913 Webster]

WordNet (r) 3.0 (2006):

upholder
    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:

106 Moby Thesaurus words for "upholder":
   Maecenas, abettor, admirer, advocate, aficionado, alpenstock,
   angel, apologist, approver, arm, athletic supporter, back,
   backbone, backer, backing, bandeau, bearer, bra, brace, bracer,
   bracket, brassiere, buff, buttress, cane, carrier, certifier,
   cervix, champion, confirmer, corset, cosignatory, cosigner, crook,
   crutch, defender, dependence, encourager, endorser, exponent, fan,
   favorer, foundation garment, friend at court, fulcrum, girdle,
   guarantor, guard, guy, guywire, insurer, jock, jockstrap, lover,
   mainstay, maintainer, mast, neck, notary, notary public, paladin,
   paranymph, partisan, party, patron, promoter, prop, protagonist,
   protector, ratifier, reinforce, reinforcement, reinforcer,
   reliance, rest, resting place, rigging, second, seconder, sectary,
   shoulder, shroud, sider, signatory, spine, sponsor, sprit, staff,
   stalwart, standby, standing rigging, stave, stay, stick, stiffener,
   strengthener, subscriber, support, supporter, sustainer,
   sympathizer, underwriter, vindicator, votary, walking stick,
   well-wisher