Alfa still supplies some parts out of their location in Orlando I think. Alfisto Steve knows much more about this, as he gets parts from them now and then, not many parts for the older cars, I suspect. They have a web site for this, but I can't find it right now in my listings, sorry.
The parts wanted/for sale category in the AlfaBB (or 164Digest for 164's) is really one of the better places to ask/search for some of the "new old stock" parts you might be looking for. Alfa owners from around the world sometimes offer such parts for sale. I once bought a "no longer available" 164 driver's inside used door handle from Oz that way.
EBay is another source for some of the parts you might want. Sometimes you might be surprised what is being offered.
There are also many new parts available from sources such as DiFatta, Alfissimo, et al, or used from APE, and many others. You never know what shows up, you just have to continue searching the web. Depends on which models Alfas you are interested in.