I bought chrome bolts from e bay. Lots of choice, lots of different lengths, and many stating that they will fit both original Alfa wheels and after market wheels.
Check the length required by measuring the thickness of the wheel boss ( the bits where the studs go through) and adding the length of the diameter of the bolt.
Ie if the boss is 25mm thick and the stud is a 12mm diameter the overall length of THREAD required is 37mm. You can get away with slightly longer but NOT shorter. Any longer and you run the risk of bottoming out on obstructions (not sure how far you can go before you do but best not to get really long bolts just in case)
Many non engineers do not realise that any threaded item reaches its full strength when screwed in the distance equivalent to its thread diameter, as stated..... 12mm bolt should be screwed
In 12mm to reach full tensile strength.
After-market wheels may require bolts with flat flanges, taper flanges or radiused flanges so you need to confirm the type of bolt required to match your new wheels. OEM Alfa wheels require radiused bolts( if my memory serves me right. I have indeed fitted my own but my mind may be playing tricks.)
Hope this all helps,