If you know any instrument repair shops locally you may be able to take the broken inner cable and they can make you a new one from the sample.
If you do buy a new cable then get one with both the inner and the outer, as the outer cable is not always the same length.
Make sure that the speedo and angle drive both spin freely. If they are very hard to turn then you will just break the new cable. Also make sure that there is no cable stub sitting in the right angle drive from the old cable, because if there is it will get pushed in when you install the new cable, it will engage enough to work, and then break within a few weeks.
James.