Mg black is indeed very easy to get, suppliers such as Daleys AMU keep vast rolls of the stuff. It is a very broad description however. There is fine and coarse varieties to choose from, and some are really supple while others are stiff (price varies). My seats came from a third party after a relationship bust up (true, he drove down the road on milkcrates she told me ). I am unsure of the material supplier, but the material is a reasonably good facsimile of the original, I am very happy with them. I wasn't going to say in case he was on here and still bitter about giving up his seats, but it's too cool a story to waste...
I know winnabbey really will chase original material, so I would speak to mini king or someone who crosses this bridge daily..
Mick