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 canâ€™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 couldnâ€™t find them.
When I bought my sons Xizang the seller had taken it to Sore Saddle Cyclery and had them add a pair of canti/v-brake bosses so junior has the best of both worlds on his Xizang as you can see in the pic, he's currently running a Ritchey V-brake but the U-brake bosses are just above it empty.
I believe that Quality Bike parts in the US still brings these in. Most any shop should be able to get them from them.