I guess you first need to be able to remove both end of the broken screw without damaging the other pieces. Then glue the broken pieces together, if it still resemble a regular screw, then go try find a match in a home hardware type store.
If it is easy to do, you may also want to disassemble the other arm to get the screw it has. It is possible that the broken screw is actually a wrong one incorrectly installed by the worker.