It's not a lens as such, it's just a right-angle mirror in an adapter that fits onto the filter ring of a lens. It's just a mirror in a tube, really. There's usually a plain piece of glass or a really cheap plastic lens element in the front of the tube to make it look more like a lens front the front.
It may be called a "right-angle mirror lens hood" or a "super spy lens"; it's very widely available and made by a number of manufacturers in a number of sizes to fit different lenses. They range in price from about $US 25 to $US 100, depending on the maker and the size. The Photodiox brand is available through Amazon. It's also available from a number of makers on eBay and from specialty online retailers like photojojo.com.