It depents on where you are living. Most Auto parts and accessories shops have wheels for a number of cars and lists so they can see whict specs your car needs.
If you want some thing original go to an Alfa Romeo dealer, their parts shop can obtain what Alfa still supply of pars and assessories and from other suppliers, too.
Internationally there are a number of independent Alfa parts suppliers, especially in England and Germany who sells wheels as well. I have always found several by Googling "Alfa parts".