John,
Four suggestions:
1 - Join the VOC
Get access to what's going on in the Club both down-under and in Europe. There are a number of dealers who have access to machines who regularly advertise in the monthly magazine MPH and you can contact them (to join the long waiting list of people wanting a Shadow unfortunately). Go to local meets, meet local riders and you never know where that might lead you in finding a Shadow.
2 - Have a new one built - 1
There is a chap who advertises in MPH to build you a brand new one to current engineering tolerances using all brand new spare parts just like the one that the Club did a couple or three years back - they run very well apparently and that one went to New Zealand so you may get the chance to see it down your way.
3 - Have a new one built - 2
Patrick Godet in France builds brand new Vincents - he can do you a Black Shadow or an Egli in a range of guises - they run really well and you can have one tailor made to suit you like a suit.
4 - Try Frank Trento's shop in Australia, he's well connected into the Vincent scene.
Good luck.
Stuart