You can sometimes get it from other people or if you go to the place where you walk with your pokemon, if you've been walking for awhile and you check your pokemon it will be holding an item and sometimes it's stuff for your fasion case. Hope it helps.
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There should be a fat guy that'll give you a fashion case after you beat the team galactic members on route 204. After you get the case the clown will be gone.
You get the fashion case after you get your first badge, and return to Jubilife City. When you leave to the north exit you will come across a situation involving Team Galactic. Once you beat them in a battle, a guy from Jubilife TV will come to you automatically and give you the Fashion Case.
play the pokeslots
In the Pokemon Fan Club, in Hearthome City.
In goldenrod city, go underground and talk to a man behind a counter. Then you dress your Pokemon up using your fashion case. There is also a salon for Pokemon and a team rocket picture.