I originally got my 3-Point replacements from World Upholstery. I checked with them and they don't seem to carry seatbelts anymore

.
This is a good place to start though. Important: Make sure that you get the ones for convertible/hardtop. Or if you want it to say Alfa Romeo on it, I checked most of the internet for them, and I couldn't find any that were direct replacements, but aftermarket replacements. If you really want them to say "Alfa" on them , go the the sale forums, someone might have them. If your car has those annoying 2-point lap belt and the shoulder belt that comes over and latches into the lap belt, get rid of them all at the same time, you'll probably have to take the seats out to get the rear seat out(to install seatbelts). Now knowing the 105 chassis, there isn't a lot of room in the back seat for passengers. I hope your rear passengers have very thin legs! Any way, I'm considering taking mine out. If I take mine out, I'll offer them to you first if you haven't already gotten some.