welcome to the forum
Recon as you have mentioned, Highwood Alfa near Swansea may also be able to help
breaker, eg GTVince has a breakers yard for 75/GTV and is bound to have some, also Vantastic on Alfaromeo75.co.uk, however I think that may still be down, but there are others being broken on there too
I have not tried this, only seen others speak of it, but an upgrade to the 75 brakes was to fit the calipers off a Volvo 240, same size but 4 pistons IIRC, all made by Brembo and a a straight swap, but that is only what I have read on a forum somewhere.
HTH and good luck