My favorite theme park gift shops in no particular order-
MK-Emporium, Merchant of Venus/Star Traders, Ye Olde Christmas Shoppe, Momento Mori
Epcot-Mouse Gear, Mexico indoor market, China's House of Good Fortune, France Souvenirs shop, UK Tea Shop and Japan Mitsokushi
DHS- The five and dime, the dark room, Legends of Hollywood, TOT gift shop
AK- The dino shop, Windtraders, Island Mercantile
Not sure if they have super unique items and what you can get only there since it varies so much but I've found lots of fun stuff at many of these shops.