I had a similar problem, went to the wrecker, he said "haven't had any parts for that type of car for a long time".
So I had a look around the yard and to my surprise found a number of cables from other cars were the same or similar. Ended up using the innner part from a Datto and my old outer.
At the time I was also looking into getting a new one made. I did a bit of searching and found a place that makes speedo cables to suit anything, you just tell them what you want. It was an aussie company, do some searching and it will turn up.