I can say with absolute certainty that your chances of finding one of those from a UK supplier are approximately the same as my chances of becoming Archbishop of Canterbury. (I am an atheist).
As you will inevitably have to import from the USA I suggest you buy from one of PSI's suppliers rather than direct from PSI. Their customer service is non-existent and their carriage charges ludicrous.
Try here: Universal Mandrel for Shopsmith: 5/8" unthreaded - WoodTurningz or here: Pen Mandrel for Shopsmith: Arizona Silhouette