The Mall doesn't sell anything. They rent spaces for stores to sell things.
You might be able to try the Halloween mega store in the mall or in the Kmart plaza
You'll have to check at the Halloween superstore over by H.L. Stephens
I did see a Slytherin adult onesie at Burlington. You can also check Hot Topics, across from Burlington. There is a seasonal costume store at the mall also.
If not at the mall there is a Halloween store
at arnot mall costume store and hot topic might look online hot topic too
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