When I got my xizang it had no brake bosses on it. The problem was that the mounts on the frame took U-brake threads (but in the right place for cantis) The U-brake has a different thread and is a larger diameter, but you can’t get cantilever arms on them because they are thicker and too short.
I ended up getting a set made up at a shop that specialises in bike engineering, it cost me about AU$100 for the set, but it seemed to be my only option (one was slightly bent) but they work fine. If there is such a brake boss that is standard with a U-brake thread and a canti mount I couldn’t find them.