Ok if it was accordingly to where I live (since idk many store's around the world I've only been in two places):
Rarity: Cache, Macy's, Sears, Deb, or that dress shop in our North Street XD
Rainbow Dash: Dick's Sporting Goods, Nike, Footlocker, The Shoe Dept., Journey's, Spencer's
Applejack: JC Penny, Hollister, And/Or order stuff online
Fluttershy: Aeropostale, Deb, Forever 2, Hot Topic (the clothes that kinda hides you like hoodies), that nature store in the mall
Twilight Sparkle: my college store, FYE, Gamestop, Ralph Lauren
Pinkie Pie: also Deb, also Spencer's, that cool store in our North Street, order stuff online etc.