A tilt-shift lens is primarily used in architectural photography to control perspective. They are not typically used in portraiture however they are great for shifting the plane of focus and creating unusual narrow depth of field portraits.
It's not a very common lens as it's quite expensive too. However I found an inexpensive solution. I bought an old 50mm Nikon manual lens. You can create this focus effect by not actually attaching the lens to the camera. Instead hold it against the camera opening and tilt the lens slightly in different directions. You'll get unpredictable results and light leaks but that adds to the fun.