rcpantera:
If the photo in your avatar is that of your car, you already have original group 4 magnesium wheels either from Campagnolo or Tecnomagnesio. If your question is about the aftermarket 17" wheels, I only know of American vendors who sell them. If I recall correctly, Pantera East, Wilkinsons and Precision ProFormance all handle them. I'm not sure if anyone sells a Group 4 wheel in 18" diameter. Do a little surfing on the vendor web sites or call them to see what they offer.
As for your question about their appearance, I have never seen a 17" reproduction that looked as dished as the originals. I doubt that it would be physically possible to duplicate the relative dimensions because your are building a taller wheel but with the same width. If the dishing on the 17" wheel was to remain in the same proportion as on the 15" wheel, the width of the wheel would have to be increased and the wheel would no longer fit your car.
In short, if you like the look of the originals, then stay with them. They are much lighter than the aluminum replacements, which increases suspension compliance and reduces rotational inertia. They also allow you to maintain a lower ride height and centre of gravity, improving your handling. Tires in the 15" size may be expensive, but they are still available. So why fix it if it ain't broke?