Since your car is '68, I assume that you have smaller ate system that has a rotor with wheel studs on it.
Centerline Alfa lists front rotor for that particular system (BR103) for $56.00. If your car happens to be one of the first cars fitted with ate system, you might have a rotor that looks identical to dunlop rotor but smaller in diameter (I had them on my '67 GTV).
Alfa Stop has pretty much anything when it comes to brake system on alfa, and they do carry dunlop rotor, but it has to be machined to decrease the diameter for very early ate system. And as you mentioned, another option would be to fit the system from 2000 series, but in order to do so, you will need a set of front spindle from 2000 gtv spider or berlina. They show up on ebay pretty often and are not expensive at all (unless you insist on getting a set of new ones).
Hope it helps,