Alfaholics sells hub and horn button separately from the wheel - at least they used to. Not cheap.
The reproduction horn button is slightly larger than the original wheel - at least it was for my wheel. So you need to machine the inner hole a bit.
Also be prepared to potentially redrill the hub, as the original Hellebore wheels are not necessarily totally identical in hole spacing.
Finally, make sure that the hub height is correct for you, as otherwise you may be adding a spacer and weld that on and remachine - there used to be several heights available from Alfa.
I did all of that and it would have been just as easy to get a machinist to make a hub from scratch ...