If you need a used one, this guy sells one from his '94 325i: http://cgi.ebay.com/ebaymotors/ws/eBayISAP...item=2437833554
Otherwise, I recomment repairing it locally (go to yellowpages.com and type alternator in the search field and your location). Many car electrical shops do that. It may be the worn-out brushes or a blown voltage regulator (30$ to repair), or DIY for 20 bucks. If diode is shot - you're looking at 80$.
I wouldn't recomment buying a rebuilt alternator from autozone.com or discount auto parts - they sell cheap remanufactured ones that don't last long (Bosch 90Amp is around 300$).
Don't forget to test your battery (take it to autozone or similar and test for free). Deffective battery can kill your alternator as well.