If your child is hoping to dress up as Frozen’s Princess Elsa or a Teenage Mutant Ninja Turtle next Friday, you’d better do some fast online shopping: The coveted costumes, fueled by recent hit movies, have been sold out for weeks at local Halloween stores. “People started grabbing them up as soon as they arrived on our shelves at the beginning of September,” says Thomas Ordaya, manager of Rockville’s Party City. It’s the same story at Target and Halloween Adventure, where disappointed little girls have been forced to choose between leaving empty-handed or picking a less popular princess. (Apparently Elsa’s sister Anna doesn’t cut it. “Barely any girls want to be Anna,” one store manager told us.) If you know a little girl who simply must be Elsa, click here to see our favorite (but pricey) version of her dress. As for the boys, it seems they’re a little less picky about their costumes: When they can’t be Turtles, many are settling for just plain ninjas.