I have two, actually. One is a sample size 2 for when I knit something for magazines, and the other one is my size. I purchased both of them from an eBay seller in New Jersey who carried a wide range of real dressmaker's mannequins covered in muslin and stamped with the dress size they represent. They each ran me about $100. I not only use them to check against size, but I stitched in thread several reference points, like where the collarbone is, neck outlines, the pants mid-fly point (a common length for my pullovers), things like that. I pin pieces to them all the time and use them almost constantly. But that's me. Mine live in full view and the space they take up isn't such a big deal because A. I have a craft room, and B. I think they are kind of pretty.
More cheaply and less deserving of a spot in the living room corner: Joann's sells an adjustable one that is not exactly attractive, but you can leave a t-shirt on it all the time to cover up its unapologetic mechanicalness. They have a 50% coupon in almost every flyer, which counts even for special order items which a mannequin might be depending on your local Joann's. And I second the duct tape version Danielle suggested. I made one with friends years ago. We made a day of it, drank daiquiris, wrapped each other with duct tape, and laughed our asses off.