eBay's Cisco offerings: http://www.ebay.com/
Search for Cisco or for the Cisco model that you are interested in. For example: Cisco 2501 or Cisco 1604. Make sure you get at least 16MB of Flash and 16MB of memory in your lab routers. Prices vary, so check a seller's comments, and by all means take your time to research the routers or switches you are interested in.