JM,
A lot of the parts are discontinued, but sometimes you get lucky, especially with ones that were used on many different models.
I enter part numbers into the BikeBandit search to find out if they're still available or not (parts with a 'Qty' window showing are still available) and also to see which models shared parts (the 'Fits machines' link). Unfortunately, BikeBandit converts the OEM part numbers to proprietary ones, so you can't pull up a diagram and get OEM numbers for searching on eBay etc. - otherwise I feel it would be the best search engine by far.
I used to like the Babbitt's search engine and used it almost exclusively, but they went and "improved" it again and now it's super slow, at least on my computer. It uses OEM numbers for everything, though:
http://www.babbittsonline.com/honda-motorcycle-partsI order almost all my parts these days on my local dealer's Web site, then pick up the parts at the store.
Ray