Sure, gag gifts are juvenile. And there’s always a chance they’ll backfire on the giver. There are some who would appreciate a pair of x-ray specs for Christmas, and some who, to put it mildly, would not. But if you’re in the market for anything from an exploding pen to a ventriloquist’s dummy, there’s no finer purveyor of toys that seem to have fallen from the back pages of an old comic book than Bernard’s Magic Shop. Founded in 1937, it also stocks costumes, stage makeup, and magician’s supplies from beginner to professional level.