costumer vs costumier

costumer and costumier both are nouns.

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

As nouns, costumer and costumier are synonyms defined as:

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

Other synonyms of costumer include costume designer.

costumer (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)