I thought you could use a land rover coupling?
Can you not get one made up or are you just after the rubber bit in the middle?
If it's the rubber bit, you can try replacing it with a metal disc, but that makes thing a bit hairy as there is no give in the steeringbwhen you hit a small bump in he road so not really recommended.

If you are struggling for a full one the granadas also used the same parts so you may be able to use that one, but it's got to be from a power steering car as the non ps columns have a different set up.