I assume you mean Windows *Phone* 8.1? Well, there's obviously any that are sold by TMoUS MVNOs (check Wikipedia or something). There's the ATIV S SGH-T899M, which was an 8.0 launch device but still has decent specs and is fully compatible... you have to get it from Canada, though. Probably any other phones from the same carrier (TELUS Mobility) will also work. I don't know of any others that are going to be fully compatible but which they don't sell; there might be a few, though.