costumier vs costumer

costumier and costumer both are nouns.

Word	Noun	Adjective	Verb	Adverb
costumier	Yes	No	No	No
costumer	Yes	No	No	No

As nouns, costumier and costumer are synonyms defined as:

costumier and costumer: someone who designs or supplies costumes (as for a play or masquerade)

Other synonyms of costumier include costume designer.

costumier (noun)	costumer (noun)
someone who designs or supplies costumes (as for a play or masquerade)	someone who designs or supplies costumes (as for a play or masquerade)