Most of the shops in both parks offer Harry Potter clothing, especially in the Wizarding World sections. The quality really is superior to what is available in retail stores.
Every gift shop in the area. You can find them cheaper at Walmart, Target, or Hot Topic; just not as wide of a selection.
Wal-Mart carries them for about half the price. Not a big selection like the parks but kids grow out of clothes so fast
In the Harry Potter World area. Every store there.
Just about any gift shop on city walk or either park should have them.
Pretty much any gifts hop in the area, or any shop on the boardwalk (before you enter the park)
Most, if not all, of the gift shops at both Universal Studios and Islands of Adventure sell shirts related to Hogwarts and Harry Potter specific logos.
Plenty of shops in there
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