I am looking: Basically everywhere
Theaters are pushing hard to bring Swifties to their theaters, and many are doing so by offering special merchandise. (Be warned, though: Some of these items may sell out.) AMC Theaters sells collectible cups ($10 for a small, $12 for a large) covered in colorful publicity photos of Swift from different “eras” of her career. There's also a popcorn box (similarly plastered with green, pink, blue, and yellow promotional photos of Swift) and a mini poster for your office cubicle.
Landmark Theaters is also offering collectible cups and buckets of popcorn for the movie. The bucket, which includes bottomless popcorn, will cost about $14, according to the theater. The refillable collector cup will cost $11. Regal Films they sell the same collectibles but for a composite price of $19.89 (yet another reference to Swift's album).
At the Alamo Drafthouse, the public can pick up a limited-edition shirt, heart-shaped sunglasses, a disco ball cup (“Alamo edition”) and friendship bracelets. The dine-in theater also offers Taylor Swift-themed drink specials to fill your cup, including a 'Bad Blood-arita', 'Ranch Water on My Guitar' and 'Prosecco Problems'.