Depends a little on the flavour of the bike. French bikes can be a joy to find parts for if they're old enough to use french threading. For headsets, I'd say it's more an aesthetic or convenience choice. If you're fixing it, the nice thing I've found about using cups and bearing bottom brackets as opposed to cassette is that you have more flexibility flipping the spindle or trying different spindle lengths to help with your chainline.
Parts: try here, many of us have spare stuff. Speaking of which, Jeremy, did those brakes work? Parts Unknown in the alley beside Segovia meats on Augusta in Kensington is an option, Bikes on Wheels on Augusta has a parts board back at the shop entrance. I'm not sure how Bike Pirates is stocked for parts. Other than that, talk to the shops with good wrenchs for suggestion (e.g. Cavern). Keith's helped me out with bits and bobs from time to time.