Chris, I think if you want E-commerce facilities there is ClassicAlfa which is a pretty good resource. There is also Centerline in America which has a good pdf catalougue you can download.
Centerline Alfa Romeo (888) 750-ALFA
However I don't personally think E-commerce is the way forward for old car bits. Unless you are just buying consumables like filters and pads there are so many anomolies between models - or even within the same model - that it is usually best to do that old fashioned thing and talk to someone first, although I appreciate that might impact your phonebill more than mine!
Personally I find Chris at Highwood excellent for a quick bit of advice and prompt delivery. He also charges postage in proportion to what he's sending which always annoys me a bit with ClassicAlfa where it's a flat fee even if you just want a gasket.
One last thing I have found useful in the past is if you don't know what the exact part reference is you need, take a photo, email it and say "I want one of these"!
