www.bikemagic.com (a sister site to this one) and www.singletrackworld.com are both good for secondhand bike bits. Try placing a 'wanted' ad, but consider e.g. Deore as well as Altus or Alivio - the aftermarket really starts at Deore and new or secondhand it can be as cheap as Altus/Alivio.
Just make sure you get something that will fit a square-taper bottom bracket.
