I haven't never heard of Alfa or some other Alfa parts dealer have rebuild manifolds. In the old days (long gone) mechanics welded cracked manifolds. If you are lucky you might find one who still can.
If this isn't possible in your case try Afra s.a.s. - News they stock lots of new old stock parts for most/all Italian car brands.
The gas engines of the F12 and A12 are series 750, 101 and 105. My best guess is the series 1 Giulietta Berlina or Sprint manifold are the most simple ones so maybe the easiest to fit the front pipe on, but I don't know exactly.
If it's the inlet manifold you shall find someone who can weld in aluminium. Again the early Giuliettas have the simplest carburettor and manifold.