costume designer vs costumier

costume designer and costumier both are nouns.

Word	Noun	Adjective	Verb	Adverb
costume designer	Yes	No	No	No
costumier	Yes	No	No	No

As nouns, costume designer and costumier are synonyms defined as:

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

Other synonyms of costume designer include costumer.

costume designer (noun)	costumier (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)