The Collaborative International Dictionary of English v.0.48:

Costumer \Cos"tum`er\ (-t?m`?r), n.
   One who makes or deals in costumes, as for theaters, fancy
   balls, etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

costumer
    n 1: someone who designs or supplies costumes (as for a play or
         masquerade) [syn: costumier, costumer, costume
         designer]