The original ones are impossible to get, but the 2nd series of Alfetta sedan (big plastic bumper version) signals fit OK. I still have some originals but they're in bad shape. I guess you can get them with the major partsresellers.
Where are you from? I fitted a pair of turning signals from a Seat Ibiza (first version). Here in Spain they're pretty cheap (7 or 8€ each) and easy to find. I think that you may be able to suit a the ones from the BMW e30 series. They are exactly the same size.