LOS ANGELES/NEW YORK—At Stationery and Toy World, a family-owned shop in New York’s Upper West Side, manager Gary Rowe is having difficulty getting all the pens and folders he ordered for the important back-to-school season. His usual vendors have low stocks of Pilot’s erasable FriXion pens and Paper Mate Flair marker pens— and prices are high on stationery and other in-demand school supplies. And Rowe is not alone, retailers are navigating a storm of challenges—higher production costs, cargo delays from China and other Asian countries, and sky-high shipping rates—as they gear up for the industry’s second-biggest selling season. “I’m hoping that when everything catches up, we get more stock,” Rowe said at his store packed wall-to-wall with a colorful array of pens and markers. “Business has been really slow.” After a year of keeping their children at home, parents are eager for classes to start again. Stimulus checks and advance …