There's a great little text story in a collection of Batman short stories which answers that question, there's a tailor shop in Gotham which retains absolute privacy and complete neutrality for it's "specialist" clients, who include a tall nameless chap who has a thing for bats, Eddie and his question mark fixation, and Selina, whose fittings he rather enjoys. What people DO with the costumes he designs and gadgets he incorporates is none of his business, and no one would harm him because it would bring everyone else down on him, besides which, he's an excellent tailor.
More recently, and in continuity, Gotham Central's "Dead Robin" arc mentioned a Gotham guy called "The Tailor", who could produce an accurate facsimilie of any costume he'd ever seen. His Robin costume reproductions were good enough to fool the GCPD when they started showing up on dead kids.